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The Great Seal Bug
Paul Reuvers & Marc Simons · 18 September 2015
Last updated 16 March 2022

The Thing
Resonant cavity microphone - wanted item

The Great Seal Bug, also known as The Thing, was a passive covert listening device (bug), in­ven­ted during WWII in the Soviet Union (USSR) by Léon Theremin (Russian: Термен) and planted in the study of the US Ambassador in Moscow, hidden inside a wooden carving of the Great Seal of the United States. It is called a passive device as it does not have its own power source. Instead it is acivated from the outside by a strong electromagnetic signal. The operating principle is based on the resonant cavity microphone, also known as an endovibrator (Russian: ендовибратор).

On 4 August 1945 1 the Vladimir Lenin All-Union Pioneer organization 2 presented a hand-carved replica of the Great Seal of the United States to US Ambassador Averell Harriman, as a gesture of friendship to their World War II ally. He hung it in the library at the US Residence Spaso House [41].

Unknown to the Americans however, the carving contained an RF radio bug of a novel design, in that it didn't have its own power source and was not connected via wires. Instead, the device was illuminated by a strong radio signal from the outside, which powered and activated it. It gave the bug a virtually unlimited life and provided the Soviets with the best possible intelligence.

The bug was finally discovered by the US State Department in 1952, three ambassadors later, during the tenure of Amb. George F. Kennan [1].

In 1951, a British radio operator had been monitoring Russian air force radio traffic, when he picked up the voice of the British Air Attaché loud and clear, but a survey of the embassy did not reveal any hidden microphones. Something similar happened to an American interceptor in 1952, when he overheared a conversation that appeared to come from the ambassador's residence at Spaso House. After a search by the Department of State, the bug was finally discovered by means of a so-called crystal-video receiver, s whilst the Russians were actively illuminating the bug [8].

Wooden carving acting as a concealment for The Thing. The rightmost image shows the position of The Thing.

The device appeared to be hidden inside the wooden carving behind the ambassador's desk, and resembled a cylindrical microphone with an antenna rod connected to it. Tiny holes in the wood under the eagle's beak, guided the sound to the membrane of the bug that was mounted just behind it. When the Russians knew that an important meeting would take place, they illuminated the bug (activating it with a strong RF beam) from a nearby building or van. 4 A receiver, tuned to the bug's resonant frequency, was then used to pick up the conversation in the ambassador's library. This way the Russians kept the device operational for no less than seven years. As the operating principle of the device was initially unknown, the Americans nicknamed it The Thing.

The discovery of the bug was kept secret for many years, until the 1960 U-2 incident [5]. On 1 may 1960, the Soviets had shot down an American U-2 spy plane over Soviet airspace, as a result of which the Soviet Union convened a meeting of the United Nations Security Council, accusing the Americans of spying. On the 4th day of the meeting (26 May 1960), in an attempt to illustrate to the council that spying between the two nations was mutual, American Ambassador to the UN, Henry Cabot Lodge, revealed the Russian bugging device, as shown in the photograph at the top.

  1. The accounts differ. The date is often quoted as 4 July 1945 [43], but also as 4 August 1945 [1]. One source [52] even suggests that the gift may have been presented during WWII, as early as 9 February 1945, during the Yalta conference.
  2. The Vladimir Lenin All-Union Pioneer Organisation, also known as the Young Pioneer organization, was a mass youth organization of the Soviet Union for children between 10 and 15, similar to the Scouting organizations of the Western world [2].
  3. A crystal-video receiver is a non-selective or aperiodic receiver [15].
  4. In some literature – e.g. in [43] – it is suggested that the device was 'illuminated' from a nearby parked van, rather than from a nearby building. In practice, a nearby building seems more realistic as a parked van would eventually attract attention, especially after seven years. As Spaso House is located in a dense part of Moscow, there would have been plenty of nearby buildings that could act as a observation/listening post.

UPDATE 16 March 2022 — The FBI has meanwhile released their original report about the Great Seal Bug, which reveals all physical details and explains exactly how it works. This report was completed on 1 December 1952 and was distributed to a selected group of people three days later [46].  Read the FBI report
Henry Cabot Lodge showing 'The Thing' at the United Nations (UN) on 26 May 1960. Copyright John Rooney/AP [17].
Henry Cabot Lodge showing 'The Thing' at the United Nations (UN) on 26 May 1960. Copyright Bettman/CORBIS [18].
1 / 2
Henry Cabot Lodge showing 'The Thing' at the United Nations (UN) on 26 May 1960. Copyright John Rooney/AP [17].
2 / 2
Henry Cabot Lodge showing 'The Thing' at the United Nations (UN) on 26 May 1960. Copyright Bettman/CORBIS [18].

How it works
The Thing is a so-called resonant cavity microphone, consisting of a resonant cavity, combined with a condenser microphone. The diagram below shows the construction of The Thing, based on various publications, observations and the original FBI report 1 [46]. It consists of a copper cylin­der with a highly polished silver-plated interior, that acts as a high-Q resonant cavity of the re­entrant type. The open end of the cylinder is closed with a thin 6.35µm membrane. At the center of the cavity is an adjustable mushroom-shaped disc with a flat surface, that can be moved to­wards the membrane. Together with the membrane, it forms a capacitive microphone. An antenna enters the cavity through an insulated hole in the side of the cylinder and is capacitively coupled.

Educated guess of the construction of The Thing, updated with information from the FBI files [46]. © Crypto Museum 2015-2022.

The device has a diameter of 24 mm and is 19 mm long. It weights ~31.4 grams (1.1 ounce). The cavity has an inner diameter of 19.7 mm and is 17.5 mm deep. The antenna is ~22.8 cm long (9"). The membrane, or diafragm, at the front of the cylinder is just 6.35 micrometers thick. The tuning post can be adjusted to increase or decrease the capacity of the mushroom. The flat face of the mushroom has machined grooves to reduce the pneumatic damping 2 of the diafragm. This is further assisted by a small hole drilled through the centre of the tuning post, and two larger holes in the back. The latter two are also used to adjust the tuning post with a special tool. According to one report [24], the distance between the mushroom and the diafragm was 230µm.

Cross section of The Thing. © Crypto Museum 2015-2022.

The drawing below was taken from the orignal FBI report of 1952, which was declassified in 2019 [46]. It provides the exact dimensions of the device as well as some interesting construction details. Interestingly, the dimensions are in cm rather than inches.  More on this subject

The dimensions of the cavity are carefully chosen so that it is resonant at a very high frequency (e.g. 1700 MHz). It is then illuminated, or exited, by a strong signal from the outside, as shown in the illustration below. Any sound in the room (speech) causes the membrane to vibrate, which decreases and increases the space inside the cavity and also the capacity between the membrane and the mushroom. As a result, the reflected signal is predominantly Amplitude Modulated (AM).

  1. Declassified by the FBI in 2019 [46].
  2. Pneumatic damping is also known as the cushion effect.

Remote activation
The weak reflected signal (i.e. the re-emitted modulated signal) had the same frequency 1 as the strong illumination signal. This poses technical restrictions on the system, such as overloading the receiver. Below are several suggestions on how this might have been solved in practice.

Activation and energizing of the bug by a strong RF signal

  1. Some papers, e.g. [16], suggest that the device re-emitted the third harmonic of the illumination frequency, but this is not the case. The FBI report [46] confirms that input and output frequencies are the same.

Using directional antennas
Receiver overloading can be solved by using directional antennas (e.g. helical or yagi antennas) and mixing part of the transmitted signal (e.g. -30dB) with the input of the receiver, in order to cancel-out the excess signal. The block diagram below show how this might have been done.

Using a circulator
The dual antenna problem can be solved effectively, by adding a 3-port circulator between the transmitter and the receiver, as illustrated in the diagram below. The circulator ensures that all transmission power is passed to the antenna and that returned energy is passed to the receiver.

In a circulator, the energy is always delivered to the next port. The energy from the transmitter (1) is delivered at the antenna (2), but the energy returned from the antenna (2) is delivered at port (3) which is connected to the receiver.  Wikipedia

Using a directional coupler
Another way to solve the dual antenna problem and avoid overloading of the receiver, is to use a directional coupler as shown in the block diagram below. It effectively separates the transmitted energy from the reflected (returned) energy. In this configuration, the directional coupler is used as a duplexer. It is known that this method was used in practice, for example with EASY­CHAIR.

It is often suggested in literature that the illumination signal was a sub-harmonic of the cavity's resonance frequency. In the frequently quoted article Lev Termen's Great Seal Bug Analyzed [16], authors Brooker and Gomez go to great length to make this a plausible theory. A sub-harmonic would be easier to generate at high power, and causes less interference between trans­mitter and receiver. In that case, an illumination frequency of 567 MHz might have been used, i.e. 1/3 of the cavity's resonance frequency 2 (1700 MHz), and the cavity behaves as a tripler.
However, in order for the cavity to generate the 3rd harmonic of the exitation frequency, it must have non­linear properties, such as thin oxidized layers between contacts – similar to a semi­conductor (diode) – or loose contacts, in which case the cavity acts as a contact generator. In the given situation however, this effect is arbitrary, and its behaviour would be difficult to predict and to reproduce. It is therefore unlikely that the Russian resonant cavity was used as a multiplier.

  1. Although it is possible to use other sub-harmonics, the 2nd and 3rd sub-harmonic are the most likely candidates as oxidised metals can easily generate the 2nd and 3rd harmonics of the input signal.

UPDATE 16 March 2022 — The original FBI report [46] about the Great Seal Bug has meanwhile been released publicly, and confirms that the re-radiated signal from the bug had the same frequency as the illumination signal. This means that no harmonics were used and that the cavity does not behave as a tripler.
The resonant cavity microphone was patented by Winfield Koch at RCA in 1941 1 [7]. The actual Russian bug was invented by Léon Theremin (27 August 1896 - 3 November 1993), born in Saint Petersburg (Russia) as Lev Sergeyevich Termen (Лев Сергеевич Термен). He is also the in­ven­tor of the Theremin, a musical instrument with a proximity sensor, as shown in the image below [6].

Theremin lived in the United States from 1927 onwards, where the theremin was produced by RCA. He returned to the USSR unexpectedly in 1938 — shortly before the outbreak of WWII — after running into financial and tax problems.

In the USSR, Theremin was first imprisoned, but was later put to work at a sharashka, a secret laboratory in the Gulag camp system, along with other detained scientists and engineers, such as Andrei Tupolev and Sergei Korolev. There he developed Buran (an infrared microphone), The Thing and several other devices for the NKVD. 2

In 1947 he was released, and in 1956 he was fully rehabilitated by the Soviets. He kept working for the NKVD (meanwhile renamed KGB) until 1966. He was a teacher at the Moscow Conservatory of Music and later became Professor of Physics at Moscow State University. After 51 years in the Soviet Union, he first visited Europe in 1989 and then the United States in 1991. Early in 1993 he made his last performance in the Netherlands. He died later that year in Moscow at the age of 97.

 Read the CIA file on Léon Theremin

  1. US Patent 223811 was filed on 14 April 1941, with a priority date of 30 September 1938. As Theremin had also been working for RCA and had left the US in late-1938, it is possible that he had knowledge of the patent when he returned to the Soviet Union (USSR). However, Termen's design uses a capacitively coupled antenna rather than a (much easier) inductively coupled one as in the patent, which could mean that he had no prior knowledge about the patent.
  2. The NKVD was the forerunner of the KGB.

The diagram below shows the theoretical equivalent of the resonant cavity microphone, which is basically a tuned circuit, consisting of a inductance and a capacitance. Part of the capacitance is variable as it is effectively a microphone. The antenna is capacitively coupled to the tuned circuit.

Equivalent circuit diagram of the Great Seal Bug

A lot has been said about the length of the antenna of The Thing. In the initial FBI investigation it is claimed that the antenna length is 1½λ [24], whilst a later CIA report specifies it as ½λ [40]. Some have claimed it should be ¼λ for the exitation frequency and ¾λ for the output frequency. It has also been suggested that it has a full wave-length (1λ). In reality, the antenna was 1½λ long.

Antenna length calculation (average)

The table above gives the resonance frequencies of the antenna, if we assume the antenna to be ideal and 9" long (22.8 cm). In practice, a correction has to be applied to compensate for the so-called end effect. If we assume this scale factor to be 0.9, this gives us a frequency between 1700 and 1800 MHz at 1½λ antenna length, which is in line with the findings of the FBI Laboratory [24]. This is also confirmed in the Final FBI report, which has meanwhile been declassified [46].

Discovery of The Thing   Spaso House
Ever since the Amerasia affair in 1945 [19], the US was suspicious of bugs being planted in US embassies abroad, especially in Moscow. Although quite a few bugs were discovered in US Embassies in Eastern Europe during the late 1940s, none had been found in Moscow since WWII.

Nevertheless, it was assumed by diplomats that the walls in Moscow had ears. The suspicions increased in the fall of 1951, when a British military officer who was monitoring Russian military aircraft traffic, suddenly heard the voice of the British Air Attaché loud and clear on his radio.

Engineer Don Bailey of the Diplomatic Wireless Service (DWS) was sent to Moscow to investigate the matter, but no bugs were found. The Russians clearly had been warned and had turned the device off. Nevertheless, Bailey reported the presence of strong radio signals when the device was in operation, which led the British to believe that the Russians — like they themselves — were experi­menting with some kind of resonance device instead of a regular transmitter [8 p. 24-25].

Shortly afterwards, a US military officer had a similar experience when he suddenly overheard a conversation that appeared to originate from the ambassador's library at Spaso House — the residence of the US Ambassador in Moscow. The matter was investigated by John Ford and Joseph Bezjian of the Department of State's security team, but they didn't find anything [20 p. 136].

Early in 1952, after George Kennan had been appointed the new US Ambassador in Moscow, Spaso House was being refurbished for him, and Soviet workers were hired to carry out the work.

Kennan believed this provided the Soviets an opportunity to plant listening devices (bugs), and ordered regular sweeps. Nevertheless, repeated technical security inspections found nothing. In September 1952, Joseph Bezjian returned to Spaso House for a more extensive search. As he believed that the Russians had removed the bugs prior to the arrival of the previous search team, he posed as a 'house guest' for three days and had his equipment sent in ahead of his arrival.

In a pre-arranged plan, the ambassador dictated an unclassified piece of text, whilst Bezjian searched the premises with his crystal video receiver. 1 Using this receiver he was finally able to locate the bug in the Ambassador's library.

The library was also used as a sitting room and, in 1947, as a temporary office by Secretary of State Marshall, during the Council of Foreign Ministers in Moscow. Marshall preferred to work undisturbed and liked the casual arrangement of the furniture. It is quite possible that the Soviets gained valuable intelligence from his presence.

When Bezjian inspected the room on 10 September 1952, 2 the signal appeared to come from the wall behind a wooden carving of the Great Seal of the United States in a corner of the room. After taking the seal from the wall, the signal disappeared and Bezjian finally realised that the bug was hidden inside the seal itself and that it was remotely (de)activated from outside the building.

After a close inspection of the wooden carving, Bezjian discovered that it could be opened and that the bug was mounted in a cut-out space inside it. At first inspection, the bug resembled a microphone with an antenna attached to it. It did not have any wires or an external power source connected to it. To prevent it from being stolen, Bezjian slept with it under his pillow that night.

The next day, the mysterious bug was sent to Washington (US), where it was handed over to the FBI for further investigation. As it was intially unclear how the device worked — it didn't have any active components — it was nick­named The Thing. The FBI passed the device to its Technical Laboratory where it was inspected by personnel of the Radio and Electric Section. The preliminary conclusions were that it was a resonant cavity microphone, operating between 1650 and 1800 MHz and that the antenna had a length of 1½λ.

Together with the Naval Research Lab (NRL), the FBI lab further investigated the device and later submitted a detailed report about the thing, that was shared with other US security agencies [46].
Joseph Bezjian, a.k.a. 'The Rug Merchant', the person who discovered The Thing. Photograph via Kevin Murray [3].

President Truman ordered the NRL to develop countermeasures equipment that would be able to detect and locate passive cavity resonators. On 3 October 1952 the NRL provided the Department of State with an interim three-piece countermeasures device that was used to check the embassy for further Resonant Cavity Microphones. They also provided suggestion for further equipment.

Although the Division of Technical Services of the Department of State's Office of Security (O:SY/T) was heavily understaffed in 1947, it has grown to 15 SY engineers by 1961. Between 1948 and 1961, these SY engineers were responsible for more than 95% of all listening devices found by all US Government agencies together [20 p. 163].

  1. A crystal-video receiver is a non-selective or aperiodic receiver [15].
  2. In the official History of the Bureau of Diplomatic Security of the Department of State it is claimed that the bug was found on 12 September. We believe this to be incorrect however, as the discovery was already mentioned in an internal FBI memo of 11 September. It is more likely that the device was discovered on the day before the memo, on 10 September 1952.

Public disclosure
Washington Evening Star · 1953
Apparently someone leaked (part of) the story to the Washington Evening Star, 1 as on 20 March 1953 it published an article with the title Secret Ears in the U.S. Embassy [48]. In the article it was claimed that in the previous year (1952) a listening device had been found inside the Great Seal of the United States in the Ambassador's room, alledgedly actived by a radio beam. The story wasn't picked up widely and the US Government did not reveal further details at the time [43 p.31].

Interestingly, the article in the Washington Star also claims that the US shared the discovery of the device with the British Embassy, where a similar device was subsequently found in the office of the British Ambassador, hidden in the tail of one of the lions aside the British coat of arms [48].

 Read the full article

  1. The Washington Evening Star, also known as The Washington Star and Washington Star-News, was a daily afternoon newspaper published in Washington D.C. between 1852 and 1981.  Wikipedia

United Nations · 1960
After the Russians had shot down an American U-2 spy plane over Soviet airspace on 1 May 1960 — an incident that became known as the 1960 U-2 incident [5] —\ they convened a meeting of the Security Council at the United Nations, claiming that the US had been spying on them. In order to demonstrate that spying was mutual, the American's decided to disclose the Soviet bug, known as The Great Seal Bug, that had been found at the ambassador's residence in Moscow 8 years earlier.

The revelation of the Russian bug attracted the attention of the international press and made the headlines during the next days and weeks. The short film above is a clip from a 1960 Universal-International News reel, retrieved from YouTube in September 2015 [9]. In this clip, Henry Cabot Lodge – US Ambassador to the United Nations (UN) – shows the wooden carving of the Great Seal, along with the listening device. According to the CIA, the device shown was a replica [43 p.31].

The revelation gave rise to the thought that the Russians were technologically more advanced than the Americans, and that America lagged behind. This was emphasised by the launching of the Russian Sputnik – the very first satellite – a few years earlier.

It has often been suggested that US officials had no idea how the device worked and that they to the UK for help [8]. There is sufficient evidence from official reports however, to assume that the various US agencies had thoroughly investigated the device and had a good under­standing of its operation [46]. The following agencies each performed their own investigations:

  • Department of State
    The device was first discovered at Spaso House by technician Joseph J. Bezjian of the Division of Technical Services of the Office of Security of the Department of State, commonly identified as O:SY/T, when conducting a sweep in Moscow, under supervision of his chief John W. Ford. He made the discovery jointly with Embassy resident personnel officer Sam Janey [47]. The discovery was immediately reported to the FBI.  More

  • FBI
    The day after the discovery, the device was flown to Washington where it was handed over to the FBI. Personnel of the Radio and Electrical Section of the FBI's Technical Laboratory then investigated the device on 16 and 17 September 1952. Their preliminary findings are presented in an internal FBI memo of 23 September and in the final technical report on 1 December 1952. By that time, the FBI had two working replicas of The Thing.  More

  • NRL
    After the initial investigation by the FBI, a more comprehensive one was conducted jointly by the FBI Technical Laboratory and the Naval Research Laboratories (NRL). This resulted in a detailed technical report that was released on 1 December 1952 [46]. Furthermore, the NRL was asked to develop suitable countermeasures equipment.  More

  • SCEL
    Based on FBI drawings supplied to them at an early stage of the investigation, the Signal Corps Engineering Laboratories (SCEL) successfully built a working cavity microphone with excellent audio performance, working on 1100 MHz. The FBI was aware of this.  More

  • CIA
    The CIA was briefed by the Department of State (DoS) shortly after the device had been discovered. They were also in the Special IIC-ICIS-CIA Committee (SC) that investigated the security threats posed by The Thing. The CIA later started its own investigation and initiated a research project under the name EASY­CHAIR, with the aim to produce devices similar to The Thing, that could be used to the advantage of the CIA.  More

  • MI5
    The British Intelligence Service MI5 was contacted by the Americans soon after the device was discovered. It is unclear which US agency was responsible for the contact with MI5, but is likely that it was the FBI. It enabled Marconi Engineer Peter Wright to develop a British equivalent of the device under the codename SATYR.  More

  • NSA
    At the time The Thing was discovered, the NSA had just been established as the successor to the AFSA. Although it is certain that NSA officials were aware of the discovery, it is currently unknown whether they conducted their own investigation.

  • Bell Labs
    During a meeting at SCEL on 15 December 1952, it was mentioned that Bell Telephone Laboratories (Bell Labs) had developed a device for modulation of a microwave carrier by changing the physical size of a wave guide section. No capacity was used and it did not appear to be of much use to the FBI at the time.
Department of State
Although many agencies are claiming their involvement in the discovery of The Thing, the device was actually found by the Department of State (DoS), during a staged undercover security sweep in September 1952. Although resonant cavities were known in the field of radar, their use as bugging devices was completely new. DoS security technician Joseph Bezjian immediately realised this, and secured the device so that it could not be stolen by the Russian staff at Spaso House.

The day after the discovery, the device was sent to Washington (USA), where it was handed over to the FBI for further investigation. The information gained from this investigation was later used to educate the various agencies in the detection and discovery of similar devices. On 5 December 1952, after the FBI was done with it, the device was returned to the DoS [33]. A former Foreign Service officer recalls it was on display in the SY's conference room in the 1960s [14].

FBI   Technical Laboratory
The first agency to investigate the device after its discovery by the Department of State, was the Federal Bureau of Investigation (FBI). The day after the device was discovered by the Department of State's Joseph Bezjian, it was flown to Washington, where it was handed over to the FBI. It was subsequently examined on 16 and 17 September 1952 by personnel of the Radio and Electrical Section of the FBI's Technical Laboratory, who reported the following on 23 September [24]:

It was found that the 'ultramodern radio transmitter' consisted of a cavity resonator with a condenser microphone built into the high impedance end of the cavity and with a 1½ wave length antenna extending out the side of the cavity. It was immediately apparent that the device was designed for use as a microphone unit without any external wire connections by beaming a UHF radio signal of suitable strength and frequency toward the antenna of the unit and by using suitable UHF radio receiving equipment which would pick up and demodulate the re- radiated and modulated signal emitted by the antenna of the device.
Determining the operating frequency of the device had not been easy as the Bureau's equipment was not suitable for frequencies above 400 MHz. Luckily, they were able to borrow the following test equipment from the National Bureau of Standards (now: NIST) for the period of one day:

  • Power oscillator, Airborne Instruments Laboratory type 124, 300-2500 MHz
  • UHF Signal Generator, Hewlett-Packard model 610A, 400-1200 MHz
  • Tuning unit, TN-19/APR-4, 975-2200 MHz
  • Tuning unit, TN-54/APR-4, 2150-4000 MHz
The most important conclusions of this preliminary investigation were:

  • Cavity resonator with built-in condenser microphone
  • 1½λ antenna length
  • Frequency between 1650 and 1800 MHz
  • Good quality of speech
  • Very sensitive, good pickup range
A day earlier, on 22 September, by special orders from the President, a Special Committee (SC) had been formed to investigate the security problems posed by the device. The SC consisted of the IIC, the ICIS and the CIA, and was chaired by Special Agent Edward S. Sanders of the FBI. At the first meeting, on 1 October 1952, all parties were briefed on the FBI's preliminary findings.

On 3 November 1952, the FBI conducted a 45 minute briefing for technical personnel of the Office of Special Investigations (OSI) 1 in which the characteristics of The Thing were explained, in the hope that they might be able to recognise the devices in the field. This is what we learn:

  • Operating frequency: 1700 MHz
  • The device has serial number 11
  • Device has been tested over a distance of 75 feet (23 metres)
  • Only one device has been found so far
  • It is unknown whether copies have been made
  • Countermeasures are being developed
By the time the final technical report was ready on 1 December 1952 (see below), the FBI had two working copies of the Russian cavity microphone. These copies may have been used by the FBI for the development of devices for their own use, but may also have been passed to other agencies. It is likely that the CIA produced its own prototypes 2 based on drawings supplied by the FBI.

Interestingly, the technical drawing that is part of the FBI report [46], shows all dimensions in cm rather than inches, which suggests that at least part of the survey was carried out in a metric country and not in the US or UK. Following the close relationships after WWII, the most likely candidates for this would have been The Netherlands and Norway. As The Netherlands can be ruled out — they were involved in the CIA research on EASYCHAIR — our best guess is Norway.

 Download the FBI report

  1. The name OSI (Office of Special Investigations) is shared by several government agencies, such as the Air Force and the Department of Justice.
  2. On page 36 of the CIA file on Léon Theremin [43] a photograph is shown of a cavity resonator which is clearly different from the one found inside the Great Seal [46]. In fact it resembles the drawing in the CIA Report on Research on EASYCHAIR [40], with suggests that the CIA had built its own prototypes.

NRL   and final FBI technical report
One of the first organisations to be involved in the investigation besides the DoS and the FBI was the Naval Research Laboratory (NRL), which worked in close cooperation with the FBI during the technical investigation. A number of FBI Special Agents were actually at work at the NRL facilities.

To avoid duplication of efforts, the NRL was ordered to develop a countermeasures receiver that was able to detect resonant cavity microphones. Although NRL didn't want to take such receivers into production, they presented a working prototype on 3 October 1952. On 1 December 1952, the final top secret FBI/NRL technical report was ready [42][46]. It consists of the following:

  1. Report of the FBI Laboratory's analysis and experiments
  2. Detailed scale drawings and photographs
  3. NRL report with design of countermeasures equipment
 Download part 1 and 2 of the FBI report

Numbered copies were distributed to the following people:

  1. President of the United States (via Matt Connelly)
  2. John W. Ford, Department of State
  3. John W. Ford, Department of State
  4. Rear Admiral Carl F. Espe, Director of Naval Intelligence (ONI)
  5. Major General Richard C. Partridge, Assistent Chief of Staff (Army)
  6. Major General Joseph F. Carroll, Director Special Investigations (ASAF)
  7. General Walter Bodell Smith, Director, Central Intelligence Agency (CIA)
  8. Interdepartmental Committee on Internal Security (ICIS)
  9. Gordon E. Dean, Chairman, Atomic Energy Commission (AEC)
  10. ?
  11. ?
  12. ?
  13. John W. ??? 1
  1. Unreadable, but possibly John W. Ford.

The Signal Corps Engineering Laboratories (SCEL) had been informed by the FBI at a very early stage. They were briefed on the current status of the investigation during a meeting of the IIC-ICIS-CIA Special Committee (SC) on 1 October 1952. When the FBI was urgently looking for a microwave transmitter, they were allowed to borrow one from the SCEL at Fort Monmouth (NJ).

After the final technical report of the FBI had been completed on 1 December 1952, the borrowed equipment was returned to the SCEL on 15 December 1952 by Special Agent Swartz of the FBI. On this occasion the FBI report was discussed along with the possibility for future work in this area.

Based on the drawings of the Russian cavity microphone that were supplied by the FBI, the SCEL had been able to built a working replica of it. The unit worked at 1100 MHz and had an excellent audio sensitivity, not least because the membrane was placed just 25 µm (1 mil) 1 from the disc. The device had been tested through a plywood wall with a low power transmitter, and produced an AM signal with approx. 50% modulation depth. No Frequency Modulation (FM) was noticed.

  1. In the original Russian design, the distance between the membrane and the mushroom shaped disc might have been larger, but this is not certain.

Shortly after The Thing had been discovered in Moscow, the Central Intelligence Agency (CIA) was briefed on the matter by the FBI. On 22 September 1952, by special orders of the US President (POTUS), a Special Committee (SC) was formed, consisting of the two internal security committees of the National Security Council (NSC) (i.e. the IIC and the ICIS) in collaboration with the CIA, to examine the internal and external security problems posed by resonant cavity microphones.

During the FBI investigation, the CIA was kept informed of the latest FBI progress, via the SC meetings. This means that they had access to the results of the preliminary FBI investigation, as well as to the final report of 1 Dec 1952 [46].

Based on this information, the CIA started its own research project with the aim to develop listening devices based on the principle of the resonant cavity microphone. The project was codenamed EASY­CHAIR and was largely carried out by a laboratory in The Netherlands, as David Wise correctly states in his book Molehunt [11]. Unfortunately, Wise did not disclose where in The Netherlands the secret research was done.

For internal and external research, the CIA had built a number of resonant cavity micro­phones that were very similar to The Thing, albeit with limited success. They were accompanied by a detailed description of their operation [40]. The image on the right was taken from that report.

Interestingly, the antenna length is specified by the CIA as λ/2 whereas, according to the FBI reports in 1952 [24][46], it had a length of 1½λ.

In September 2015, Maurits Martijn and Cees Wiebes revealed in the Dutch online magazine De Correspondent that EASYCHAIR had been carried out at the Dutch Radar Laboratory (NRP) in Noordwijk [13]. Between 1955 and 1992, the NRP produced a wide range of innovative covert listening devices (bugs) for the CIA, starting with the Easy Chair Mark 1, or EC Mk 1, in late 1955.

In total, five generation of passive Easy Chair devices were made, before CIA and NRP transitioned to active listening devices. The active CIA/NRP bugs are characterised by their high audio quality, low probability of detection (LPD) and low probability of intercept (LPI). Many of the original passive and active EASY­CHAIR devices are now in the collection of Crypto Museum.

 More about EASYCHAIR
 Read the CIA Report on Research on EASYCHAIR

In his book Spy Catcher [8], former MI5 scientific officer Peter Wright gives an account of how he was approached by MI5 in 1952 1 to find out how The Thing worked. Apparently the CIA and FBI had no idea of the operating principle behind it and had presented the device, or a replica of it, to MI5. Wright, who was still employed at the Marconi Company in Great Baddow (Essex, UK) at the time, reverse engineered the device after work hours. It took him ten weeks to solve the puzzle.

Once Wright had figured out the principle, he was able to demonstrate it to his contact at MI5.

In the next 18 months he developed a British equivalent that would become known as SATYR. Using two British umbrellas as transmit and receive antennas, Wright called it Black Magic.

Several complete SATYR sets with matching transmitters and receivers, were ordered by MI5 and were subsequently used by the British, Australian and Canadian intelligence services throughout the 1950s and probably the 1960s.

According to Wright's own account, the CIA ordered twelve complete sets and rather cheekily copied the drawings to make twenty more themselves. According to Wright, the American variant was called EASY­CHAIR (a.k.a. Mark 2 and Mark 3). This is contradicted by former CIA technical officer Peter Karlow however, who states that MI5 never shared its discovery with the CIA [11].

It is currently unclear which American agency was responsible for the contact with MI5 and/or Peter Wright. Although it is often assumed that it was the CIA, it might also have been the FBI or the State Department. If Wright had been given the original device during the 2nd half of 1952, his contact was most likely the FBI as it was in their custody from 12 September to 5 December 1952. But they could also have given him a replica, as it is certain that the FBI had several of them by 1 December 1952. In any case, this would explain why, according to Karlow, the CIA was not aware of any work carried out in the UK [11].

As the CIA was frequently updated by the FBI during the investigation, via the IIC-ICIS-CIA Special Committee (SC), it is also possible that the CIA's Technical Services Division (TSD) was able to build a functional replica at an early stage, and that this replica was supplied to MI5 and Wright. However, in that case the CIA must have been aware of the British research and its outcome.

There are indications that the British were indeed briefed on the subject, as confirmed in (partially declassified) letters from the FBI office in London (UK) of 6 October, 24 October [28] and finally of 8 December 1952 [35]. The latter also suggests that the British had their own research, as they promised the FBI to keep them informed of any local developments as part of their cooperation. If this is the case, it is likely that Wright was contacted (via MI5) by the FBI rather than by the CIA.

Wright had probably not been given the full picture by American and British intelligence when they hired him. It is likely that similar research projects, carried out by other parties, were kept from him, leading him to believe that he was the only one to solve the case for the Americans. It is possible that Wright solved the mystery for the FBI, and that the FBI did not share this with the other agencies. In the Final FBI Report [46] it is stated that the membrane of the original device was ruptured and had to be replaced. Wright claims exactly the same in his book Spy Catcher [8].

 More about SATYR

  1. In 1952, Peter Wright was officially employed as a Navy scientist, attached to the Marconi Company at Great Baddow (Essex, UK) working under government contract on naval radar systems. He was frequently consulted in secrecy by MI5. In 1954 he became the first principal scientific officer of MI5 [8].

US Countermeasures
Immediately after the discovery of the Russian resonant cavity microphone, the FBI started an all-out investigation; one of the most difficult projects in their history [34]. At the same time, the Naval Research Laboratories (NRL), who were helping the FBI with their research, were ordered to develop a countermeasures receiver that could be used to find resonant cavity microphones.

During the course of the development, part of the technical staff of the FBI Laboratory worked at the NRL facilities in Washington. In particular Ivan Conrad and Special Agents Sutton, Swartz and Bradley, were commended for their excellent technical contributions, as well as the NRL team supervised by Captain Beltz. They had completed their work in a very short time frame.

Details of a suitable countermeasures receiver are included in the final FBI/NRL report [46]. On 3 October 1952, one countermeasures set was presented to the State Department, consisting of:

  1. Transmitter (generator) for the 65 - 3000 MHz range
  2. Sound source to excite the microphone (tone generator)
  3. Especially developed receiver (described in NRL Report #4087)
As the FBI/NRL considered that resonant cavity microphones could be used practically between 65 and 3000 MHz, they felt it was necessary to search that entire frequency spectrum for bugs of this type. The transmitter listed under (1), consisted of three separate units:

  • General Radio type 1208A oscillator (65 - 500 MHz)
  • General Radio type 1209A oscillator (250 - 900 MHz)
  • Oscillator part of an R-111B/APR-5A military receiver (1000 - 3000 MHz)
To sweep an area for resonant cavity microphones, a sweep team could deploy all three parts of the system (transmitter, tone generator and receiver), but this would reveal their presence to the eavesdropper. It was considered better practice to wait until the bug was activated by the eaves­dropper, after which only the receiver was needed to locate the bug.

Although the NRL developed a suitable countermeasures receiver, and provided a prototype on 3 October 1952, they were not equipped for series production such devices. It was there­fore decided that the Atomic Energy Commission (AEC) would build the actual receivers, but getting a price quotation from them, and determining how many receivers should be ordered, appeared to be difficult [38]. Apart from the receivers, special microwave generators, manufactured by the General Radio Company, were required in order to activate any cavity microphones [31].

In any case, the FBI did not posess a complete countermeasures set by January 1953, as they were unable to allocate the necessary funds for purchasing the required generators in December 1952 [32]. When the White House asked the FBI to perform a security survey of the White House in January 1953, they had to reconsider their decision for not buying the generators [39].

Soviet countermeasures
Being the first users of resonant cavity microphones, the Russians realised that, once the devices were discovered, they would be copied by Western agencies, and that sooner or later they would be used against them­selves. The Soviets therefore developed their own countermeasures.

An example of a Russian countermeasures receiver that was developed especially for the detection of resonant cavity microphones, is the OSOBNJAK 8 shown in the image on the right.

The device is housed in an unobtrusive briefcase and can detect strong nearby radio signals between 100 MHz and 12 GHz; the signals that are typically used to activate such microphones.

 More about Osobnjak 8

In most cases, it will be sufficient to know that a strong microwave signal is present, so that the meeting can be moved to a different location, or can be cancelled altogether. In some cases however, the bugged party might want to know who is eavesdropping on them, and from which location the activation beam is sent. The suitcase is not suitable for direction finding however.

In order to find the location of the illumination signal (i.e. the activation beam), one would need an aperiodic receiver 1 for the suspected band, plus an antenna with a narrow viewing angle.

A suitable solution is the MRP-4 radar locator, developed by Tesla in Czechoslovakia in 1972. This device can be worn on the chest, with the antennas facing forward. Although originally developed for finding radar stations, it can be used for locating any nearby strong transmitter. In addition it can be used to locate weak pulsed transmissions from radar and bugging devices.

The antennas of the MRP-4 have a very narrow viewing angle, typically between 1 and 2 degrees, making it very easy to determine the direction to the transmitter. The device is suitable for the 1 - 10 GHz frequency range, divided over four bands. Similar devices were developed in the USSR.

 More about the MRP-4

  1. Also known as a crystal video receiver.

The lauch of the Russian Sputnik — the first satellite ever — and the discovery of The Thing made it painfully clear that in some respects, the Russians were technologically more advanced than the US. From the side of the Americans, this led to a range of initiatives to understand and improve the Russian technology, and eventually use it against the Russians. Here are some examples:

In 1954, the CIA launched a research program under the name EASYCHAIR — first mentioned in 1992 by David Wise in his book Molehunt [11]. The research was done in The Netherlands by the Dutch Radar Laboratory (NRP) and led to the development of five generations of passive listening devices, named EASYCHAIR Mk. I to V.

In 1958, the EASYCHAIR technology was used to eavesdrop on the Russian Embassy in The Hague (Netherlands) for more than six months.

 More information


One of the first people to investigate the Russian resonant cavity (or a copy of it) on behalf of MI5, was British engineer Peter Wright. It took him 10 weeks to discover the operating principle.

In the following year (1953), he developed a similar system for MI5 under the codename SATYR. It used modified British umbrellas as the transmit and receive antennas.

 More information


Pulsed cavity
The initial research goal of the EASYCHAIR program was to develop a reliably operating cavity resonator – similar to The Thing – but this turned out to be more difficult than anticipated.

In 1965, the Dutch Radar Laboratory (NRP) finally succeeded in creating a reliably operating and reproduceable cavity resonator, by driving it with pulses — similar to a radar system.

 More information


By 1965, the CIA had lost interest in passive devices – mainly due to the strong activation beams involved – and shifted attention to low-power active listening devices, such as the SRT-52 shown in the image on the right.

In 1987, the Russians showed a series of liste­ning devices (bugs) that they had found in the walls of their Washington embassy. We could identify at least one of these devices as an EASYCHAIR SRT-56, working at 1500 MHz.

 More on Post-EASYCHAIR development
 The Russian press conference of 1987


Université de Bordeaux
In 2021, a group of researchers at the University of Bordeaux (France) succeeded in creating a working reproduction of The Thing, based on the drawings in the FBI report of 4 December 1952 [46], complete with an accurate reproduction of both halves of the wooden seal itself [50].

The device was first demonstrated to the public during the WPW 2022 conference held from 4 to 9 July 2022, as shown in the image on the right.

Jacek Lipkowski
In this video, posted on 1 December 2022, Polish radio amateur Jacek Lipkowski (callsign SQ5BPF) demonstrates a working replica of The Thing, using standard parts from a local hardware store.

He dimensioned the device in such a way that it operates in the 23 cm HAM radio band at 1267 MHz, which is relatively close to the original bug. Transmitter and receiver are at 90° angles [51].

 Background (off-site)
 See how it is constructed (off-site)

Neil Smith
In this video, published on 21 March 2023 [52], Neil Smith shows how he recreated the resonant cavity microphone for the BBC show The Great Genius of Modern Life with Professor Hannah Fry.

Neil Smith is the host of the YouTube channel Machining and Microwaves. It a future video he will be showing detailed information on how he reproduced the bug.

 Watch on YouTube
 Neil's YouTube channel


The following abbreviations and expressions are used in connection with this page:

AEC   Atomic Energy Commission
CIA   Central Intelligence Agency
DIRFBI   Director of the FBI
At the time of the event, this was J. Edgar Hoover.
DoS   Department of State
 Wikipedia  Website
EASYCHAIR   CIA codename
Research project, initiated by the CIA in 1954, to develop covert listening devices based on the experiences with The Thing. The codename Easy Chair was also written as EASYCHAIR or EC. The actual research was carried out by the NRP Laboratories in The Netherlands.  More
FBI   Federal Bureau of Investigation
IIC   Interdepartmental Intelligence Conference
Resides under the NSC.
ICIS   Interdepartmental Committee on Internal Security.
Resides under the NSC.
JUNE   FBI codename
The codename JUNE was used by the FBI for information about covert microphones found in US Embassies abroad, from 1947 onwards. The discovery of The Thing in 1952 is documented in the FBI archives under the codename JUNE as well.
LOSS   US codename for 'The Thing'
According to Keith Melton [10], the American codename for the Soviet covert listening device, a.k.a. 'The Thing', was LOSS.
MI5   Military Intelligence 5
British internal intelligence agency.  More
NRL   Naval Research Laboratory
Technical research laboratory of the US Navy.  Wikipedia
NSC   National Security Council
O:SY/T   Office of Security - Technical Services
Division of Technical Services (T) of the Office of Security (O:SY) of the Department of State.
POTUS   President of United States
RAINDEER   Soviet codename
Russian codename for The Thing (Северный олень).
SA   Special Agent
Common expression used for Special Agents of the FBI.
SC   Special Committee
Temporary commission, established by the President of the United States, consisting of the two internal security committees of the National Security Council, the IIC and the ICIS, in collaboration with the CIA, tasked to examine the security problems posed by The Thing. Also known as the IIC-ICIS-CIA committee.
SCEL   Signal Corps Engineering Laboratories
Technical engineering laboratory of the US Army Signal Corps, based at Fort Monmouth (New Jersey, USA).
SY   Security
Security engineer of the Division of Technical Services of the Office of Security (O:SY/T) of the Department of State.
The following people are frequently mentioned on this page and/or the documents that are referred to in the text. This list is by no means complete.

Belmont, Alan H.   Joined the FBI in 1936. At the time of the discovery of The Thing he was the head of the Domestic Intelligence Division of the FBI in Washington.
Bezjian, Joseph J.   Technical Engineer at the Division of Technical Services (T) of the Office of Security (O:SY) of the Department of State (O:SY/T) who actually discovered The Thing in the ambassador's study at Spaso House in September 1952. Nicknamed: The Rug Merchant.
Ford, John W.   Head of the Division of Technical Services of the Office of Security of the Department of State (O:SY/T).
Conrad, Ivan W.   Head of Division 7 of the FBI's Technical Laboratory.
Harbo, R.T.   Assistent Director of the FBI. Commonly referred to as Mr. Harbo.
Hoover, John Edgar   Director of the FBI. Commonly referred to as DIRFBI or the Director.
Ladd, D. Milton   Joined the FBI as an agent on November 1928. At the time of the discovery of The Thing he was Assistant to the Director of the FBI (i.e. the number 3 of the FBI).
Wright, Peter   British Navy scientist, attached to the Marconi Company, who worked part-time for the British intelligence service MI5. He later became MI5's first Principal Scientific Officer.
In order to help putting this complex story together, we have used the following chronological list of events as a guide:

4 Aug 1945   Great Seal Bug planted at Spaso House
The Vladimir Lenin All-Union Pioneer Organisation presents a hand-carved replica of the Great Seal of the United States to US Ambassador Averell Harriman, who hangs it in the study of his Moscow Residence Spaso House.
8 Sep 1952   Suspected penetration of the US Embassy in Moscow
Letter from John Edgar Hoover to John W. Ford, about the suspected penetration of the US Embassy in Moscow (partly released) [23].
10 Sep 1952   Discovery of The Thing 1
An ultra modern listening device is discovered by US Department of State technician Joseph Bezjian, whilst conducting a pre-arranged sweep at Spaso House, the residence of the US Ambassador in Moscow.
11 Sep 1952   FBI internal memo about discovery of The Thing
Internal FBI memo from AH Belmont to DM Ladd (not released).
15 Sep 1952   The Ting received by the FBI
The device is handed over by the Department of State to the FBI for a technical investigation.
16-17 Sep 1952   Preliminary investigation by the Technical Laboratory of the FBI
The preliminary findings are reported on 23 September 1952.
22 Sep 1952   Formation of IIC-ICIS-CIA Special Committee
The President orders the creation of a Special Committee (SC), consisting of the IIC, the ICIS and the CIA. The SC has the task to examine security problems related to the discovery of The Thing.
23 Sep 1952   Internal memo with preliminary findings
Internal FBI memo from IW Conrad to Mr. Harbo (partly released) [24].
1 Oct 1952   First IIC-ICIS-CIA Meeting (SC)
Meeting in which the chiefs of staff are briefed on the topic. Present are representatives of the Army, Navy, Air Force and the National Security Council. SA Edward S. Sanders is later appointed chairman of the SC.
3 Oct 1952   First countermeasures
The Naval Research laboratory presents a prototype of a suitable countermeasures setup that can be used to detect and locate any resonant cavity microphone in the 65 - 3000 MHz frequency range.
6 Oct 1952   Progress report to POTUS
The Director FBI informs the President of the United States about the excellent technical progress that has been made by the FBI and the NRL.
13 Oct 1952   Development of 'resonant cavity'
The FBI wants to advise the Domestic Intelligence Division on how soon resonant cavities and their activating devices can be made available for their own purposes.
15 Oct 1952   POTUS asks FBI to inform the SC
In the light of the technical progress that was reported on 6 October, POTUS asks the FBI to keep the IIC-ICIS-CIA committee informed (partly released). [25].
23 Oct 1952   Request for OSI briefing
The Chiefs of Staff suggest that it might be a good idea to brief their overseas technical personnel, so that they can recognise the devices in the field (partly released) [26].
24 Oct 1952   Briefing in the UK
Report from the Legal Attaché in London to the Director of the FBI, about discussing The Thing with the British Government (partly released) [28].
28 Oct 1952   Visit of UK representative to FBI
Following the briefing of the UK Government on 24 October, a representative of the British Government has visited the FBI in relation to the device.
3 Nov 1952   Briefing of the OSI
Briefing of technical personnel of the Office of Special Investigations (OSI) of various government agencies, conducted by Special Agent R.W. Swartz of the FBI. During this 45 minute briefing, the FBI explained the technical characteristics of The Thing to the OSI members (released) [29].
12 Nov 1952   Great Seal provided to the FBI
The Technical laboratory of the FBI receives the wooden seal in which The Thing was was concealed, from the Security Division of the Department of State.
1 Dec 1952   Final technical report ready
Detailed technical report about the investigation of the device, jointly carried out by the FBI and the NRL.  Read the report
4 Dec 1952   Distribution of final report to controlled group
The report is made available to the President of the United States, Mr. John Ford of the Department of State, the various members of the Interdepartmental Committee on Internal Security, the Director of the CIA and the Chairman of the Atomic Energy Commission.  Read the report
5 Dec 1952   Cavity microphone returned to Department of State
After finishing the technical report on the cavity microphone, the FBI returns the Great Seal of the United States with the hidden Russian resonant cavity microphone to the Department of State (released) [33].
8 Dec 1952   Local developments in the UK
Through the US Embassy in London, the FBI is advised of local British developments in this area (partly released) [35].
15 Dec 1952   Test equipment returned to SCEL
After finishing the investigation, the equipment that was borrowed from the Signal Corps Engineering laboratories (SCEL) is returned by FBI Special Agent Swartz. On this visit, SCEL engineers demonstrate a working cavity microphone (released) [37].
16 Jan 1953   IIC-ICIS-CIA Special Committee meeting
Discussion about the production of countermeasures equipment by the Atomic Energy Commission (AEC) (partly released) [38].
12 Feb 1953   White House request for sweep
The White House asks whether the FBI is equipped to make a security survey of the White House for the detection of cavity microphones (released) [39].
24 April 2019   FBI report declassified
The official FBI report about the resonant cavity microphone is officially declassified and approved for released . This does not include the Naval Research Laboratory Report #4087.  Read the report
  1. In the official history of the Department of State, it is claimed that The Thing was found at Spaso House on 12 September 1952. This is believed to be incorrect however, as its discovery is discussed in an internal FBI memo as early as 11 September 1952 [24].

  • Type
    Passive listening device
  • Principle
    Resonant cavity microphone
  • Concealment
    Wooden carving of Great Seal of the United States
  • Inventor
    Leon Theremin (Lev Sergeyevich Termen)
  • User
    NKVD (KGB)
  • Planted
    4 August 1945
  • Discovered
    10 September 1952
  • Serial number
  • Frequency
    ~ 1700 MHz
  • Distance
    23 metres (at least)
  • Weight
    31.4 g (1.1 ounces))
  • The Thing
  • The Great Seal Bug
  • Russian Resonant Cavity Microphone
  • LOSS
  • Raindeer
  1. Drawing and Photographs, Russian Resonant Cavity Microphone  
    FBI, 1 December 1952. Released to a selected group on 4 December 1952.
    Declassified and approved for release by the FBI on 24 April 2019 persuant to E.O. 13526.

  2. FBI file on Russian Cavity Microphone
    Federal Bureau of Investigation. Various dates, redacted.
    Released under the FIOA (FOI/PA# 1173422-1) on 11 May 2014.

  3. Report on Research on EASYCHAIR
    Central Intelligence Agency - Contracting Group, 14 July 1955.
    Classification status unknown. Not marked as secret.

  4. Benjamin B. Fisher, Leon Theremin - CIA Nemesis
    CIA historical article on Theremin. Date unknown, but probably mid-2002.
    Partly declassified on 28 December 2010 and 10 September 2014.
  1. Wikipedia, The Thing (listening device)
    Retrieved September 2015.

  2. Wikipedia, Vladimir Lenin All-Union Pioneer Organization
    Retrieved September 2015.

  3. Kevin D. Murray, The Great Seal Bug Story
    Retrieved September 2015.

  4. Anonymous contributor 2, Description of the Great Seal Bug
    Former US Army Sergeant and radar repairman.
    Retrieved from [3], September 2015.

  5. Wikipedia, 1960 U-2 indicent
    Retrieved September 2015.

  6. Wikipedia, Léon Theremin
    Retrieved September 2015.

  7. Winfield R. Koch, US Patent 2238117
    Ultra high frequency modulator (resonant cavity microphone).
    30 September 1938. RCA Corporation.

  8. Peter Wright, Spycatcher
    1987-1988. ISBN 0-440-29504-1. pp. 24-xx.

  9. YouTube, UN Spy Debate. Reds 'Bugged' American Embassy Lodge Claims.
    1960. Published 14 February 2014. Retrieved September 2015.

  10. Robert Wallace & H. Keith Melton, Spycraft
    ISBN 978-0-55382-007-2. p. 165.

  11. David Wise, Molehunt
    10 March 1992. ISBN 978-0394585147.

  12. Passive Resonant Cavity & 'Spycatcher' Technical Surveillance Devices
    Website: GBPPR Homebrew Military & Espionage Electronics. Visited October 2015.
    Website no longer avaialable in 2022.

  13. Maurits Martijn & Cees Wiebes, Operatie Leunstoel
    De Correspondent. 24 September 2015.

  14. Anonymous contributor (former US Service Officer), Description of the Great Seal Bug
    The Great Seal Bug Story, Part I. Compiled by Kevin D. Murray.
    29 April 2012. Retrieved September 2015.

  15. W.E. Ayer, Characteristics of Crystal-Video Receivers Employing R-F Preamplification.
    Stanford University. Technical report No. 150-3. 20 September 1956. Obtained via [16]
    Declassified uder DoD Directive 5200,20.

  16. Graham Brooker & Jairo Gomez, Lev Termen's Great Seal Bug Analyzed
    November 2013. IEEE A&E Systems Magazine. pp. 4-11.
    This article has several issues and should not be considered accurate. It is often quoted in other publications, but the assumptions and conclusions in the article are incorrect.

  17. John Rooney (AP), Photograph of Henry Cabot Lodge showing 'The Thing'... the United Nations (UN) on 26 May 1960.
    26 May 1960. Retrieved February 2014.

  18. Bettman (CORBIS), idem
    26 May 1960. Retrieved September 2015.

  19. Wikipedia, Amerasia
    Retrieved December 2015.

  20. US Department of State, History of the Bureau of Diplomatic Security
    Global Publishing Solutions, October 2011. First Edition. pp. 161-163.

  21. Federal Bureau of Investigation (FBI), FBI file on Russian Cavity Microphone
    Released under the FIOA (FOI/PA# 1173422-1) on 11 May 2014.

  22. Director of the FBI to George C. Marshall, Secretary of State
    19 February 1947. Released under the FIOA (FOI/PA# 1173422-1) on 11 May 2014 [21].

  23. John Edgar Hoover to John W. Ford, Suspected penetration of US Embassy
    8 September 1952. Released under the FIOA (FOI/PA# 1173422-1) on 11 May 2014 [21].

  24. IW Conrad to Mr. Harbo, Results of laboratory examination
    23 September 1952. Released under the FIOA (FOI/PA# 1173422-1) on 11 May 2014 [21].

  25. Matthew Conelly to J Edgar Hoover, Request to inform IIC-ICIS-CIA committee
    15 October 1952. Released under the FIOA on 5 August 2015.

  26. AH Belmont to DM Ladd, Request for OSI briefing
    23 October 1952. Released under the FIOA on 5 August 2015.

  27. DM Ladd to Director FBI, Assignment of SA Sanders as chairman of IIC-ICIS-CIA Special Committee
    23 October 1952. Released under the FIOA on 5 August 2015.

  28. Legal Attaché London to Director FBI
    24 October 1952. Released under the FIOA (FOI/PA# 1173422-1) on 11 May 2014 [21].

  29. IW Conrad to Mr. Harbo, Briefing of OSI personnel
    4 November 1952. Released under the FIOA (FOI/PA# 1173422-1) on 11 May 2014 [21].

  30. Director FBI to Legal Attaché London
    5 November 1952. Released under the FIOA (FOI/PA# 1173422-1) on 11 May 2014 [21].

  31. IW Conrad to RT Harbo, Countermeasures equipment
    2 December 1952. Released under the FIOA on 5 August 2015.

  32. Executives Conference to Director FBI, No funds for building countermeasures equipment
    3 December 1952. Released under the FIOA on 5 August 2015.

  33. FBI receipt, Cavity microphone and Great Seal of the United States handed over
    5 December 1952. Released under the FIOA (FOI/PA# 1173422-1) on 11 May 2014 [21].

  34. DJ Parsons to Mr. Harbo, Letters of commendation
    5 December 1952. Released under the FIOA on 5 August 2015.

  35. Legal Attaché London to Director FBI
    8 December 1952. Released under the FIOA (FOI/PA# 1173422-1) on 11 May 2014 [21].

  36. DJ Parsons to Mr. Harbo, Return of equipment to US Army Signal Corps
    8 December 1952. Released under the FIOA (FOI/PA# 1173422-1) on 11 May 2014 [21].

  37. IW Conrad to RT Harbo, Equipment returned to SCEL
    19 December 1952. Released under the FIOA (FOI/PA# 1173422-1) on 11 May 2014 [21].

  38. AH Belmont to Mr. DM Ladd, Countermeasures equipment from the AEC
    22 january 1953. Released under the FIOA on 5 August 2015.

  39. WI Conrad to Mr. Harbo, White House security
    13 February 1953. Released under the FIOA on 5 August 2015.

  40. CIA Contracting Group, Report on Research on EASYCHAIR
    14 July 1955. Classification status unknown. Not marked as secret.

  41. Wikipedia, Spaso House
    Retrieved january 2016.

  42. Director FBI to POTUS (via Matt Connelly), Release of FBI report on Russian microphone
    4 December 1952. Released under the FIOA (FOI/PA# 1173422-1) on 11 May 2014 [21].
    The actual report is now also available as well [46].

  43. Benjamin B. Fisher, Leon Theremin - CIA Nemesis
    CIA historical article on Theremin. Date unknown, but probably mid-2002.
    Partly declassified on 28 December 2010.

  44. IW Conrad to Mr. Harbo, Security survey of the White House
    8 May 1953. Partly released under the FOIA on 3 September 2010.

  45. David Pursglove, How Russia Spy Radio Works
    American Electronics Illustrated, January 1962, pp. 89—91.
    Declassified and approved for release by the CIA on 25 April 2013.

  46. J. Edgar Hoover to John W. Ford
    Drawing and Photographs, Russian Resonant Cavity Microphone
    FBI. 1 December 1952. Released to a selected group on 4 December 1952.
    Declassified and approved for release by the FBI on 24 April 2019 persuant to E.O. 13526.

  47. Mark Nixon, 'The Thing' and I
    Center for Cryptologic History, 4 October 2018.

  48. Constantine Brown, Secret Ears in U.S. Embassy
    Washington Evening Star, 20 March 1953, p. A-9.

  49. Looking towards the future: the changing nature of intrusive
    surveillance and technical attacks against high-profile targets

    Thesis 2020, Imperial College London, Department of Computing.
    Jonathan Gudgeon, November 2019.

  50. Prof. Simon Hemour, Personal correspondence
    Working reproduction of the Great Seal Bug, based on [46]. 17 December 2021.

  51. Jacek Lipkowski, The Great Seal Bug Replica Demo
    Working replica of The Thing on 1267 MHz (23 cm ham radio band).
    YouTube channel sq5bpf, 2 December 2022.

  52. Neil Smith, No Wires, No Batteries - Spying Changed FOREVER because of this invention!
    YouTube channel Machining and Microwaves, 21 March 2023
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