Emil Klaus Fuchs was a German citizen who joined the German Communist Party in the 1930s but was forced to flee to Britain in 1933 when the Nazis came to power. He earned a doctorate in physics from Edinburgh University and obtained a teaching post at the university. In 1939, when the Second World War broke out, he was detained on the Isle of Man and later sent to Canada as the citizen of an enemy nation. He was eventually released and returned to Edinburgh in 1941, where his scientific expertise led to him being recruited to join the TUBE ALLOYS nuclear weapons programme - Britain's equivalent of the Manhattan Project.
In late 1941, following the German invasion of the Soviet Union, Fuchs contacted the exiled German Communist Jurgen Kuczynski to offer the Russians information on the TUBE ALLOYS project. He was put in touch with a contact in the Russian GRU military intelligence agency, to which he passed secret atomic research.
Fuchs later stated that he had been motivated by a belief that the Soviets had a right to know about the atomic bomb project. Dick White, who became Director General of MI5 in the 1950s, contrasted Fuchs' motives with the desire for money that motivated many other spies and concluded that his motives "were relatively speaking pure. A scientist who got cross at the Anglo-American ploy in withholding vital information from an ally fighting a common enemy."
Fuchs was given the codename REST and was transferred in 1942 to another GRU agent, Ursula Beurton (codenamed SONYA). The two met regularly in Banbury, Oxfordshire where Fuchs passed secret documents to Beurton. In 1943, Fuchs was sent to join the Manhattan Project as part of a British team of scientists. He played a key role in the project over the next three years, developing many of the designs, equations and techniques used to build the first atomic bombs.
During this period, Fuchs was in contact with a GRU agent whom he knew only as "Raymond". The agent was an American citizen called Harry Gold, codenamed GUS ("goose" in Russian), who had been working for the Soviets since 1934. The two met in a number of locations, including New York City and Santa Fe, New Mexico, where Fuchs provided information on technical issues relating to the design of the atomic bomb. When combined with information from other sources, this helped the Russians to make rapid progress in developing what was effectively a copy of the American atomic bomb design.
When Fuchs returned to the UK in 1946, he was offered a prestigious post at the UK Atomic Energy Research Establishment at Harwell, Oxfordshire, where he worked on developing nuclear energy. The establishment's importance and secrecy was such that it was nicknamed "the holy of holies". Security was tight; MI5 carried out an investigation into Fuchs that reviewed his record, including pre-war allegations of communist activity, but found nothing incriminating. However, Fuchs continued spying until he was finally exposed in 1949.
Fuchs was eventually caught out by a breach in Soviet security. Since 1943, the UK and United States had been working on a project codenamed VENONA to break the Soviets' secret codes. Poor Soviet security practices enabled US codebreakers to decipher a large number of messages, some of which were reports of Fuchs' meetings with Gold.
It was clear that there was a major Soviet spy in the Manhattan Project. US and UK investigations focused on identifying the individual concerned. Although it was not clear at first that Fuchs was the agent known as REST, investigators were able to narrow the suspects down to only two people - Fuchs himself and a fellow German-born scientist, Rudolf Peierls. By 1949, it had become clear that Fuchs was the mole.
It was not possible to use the VENONA material to convict Fuchs - due to the extreme secrecy of VENONA, the intercepts could not be disclosed in court. From July 1949, MI5 intercepted Fuchs' mail and telephone calls but found nothing incriminating. It was decided that pressure would be put on Fuchs to induce him to confess.
William "Jim" Skardon, a former police Special Branch officer, was chosen for the task. He was given an opening when Fuchs' father accepted a post at the University of Leipzig in communist-ruled East Germany. This was a potential security problem for Fuchs, who had already discussed the issue with the authorities at Harwell. Skardon was able to use this as a pretext to arrange a series of meetings with Fuchs to discuss the scientist's personal life.
Skardon gradually gained the confidence of Fuchs over a period of several months. He finally confronted Fuchs in December 1949 with the news that his involvement in espionage was known to MI5. The scientist initially denied the charge, but a month later he told Skardon that his conscience had compelled him to come clean.
Fuchs gave his interrogator a lengthy though somewhat partial confession. He admit that he had spied for the Soviets since 1942 and had given them crucial secrets on the atomic bomb project. At the same time, however, he consistently refused to divulge some aspects of his work as a spy. After a short trial in which he pleaded guilty to charges of breaching the Official Secrets Act, he was given the maximum sentence - fourteen years' imprisonment. After his release he went to East Germany, where he died in 1988.
The story of the Fuchs case is covered in The traitors: The double life of Fuchs, Pontecorvo, and Nunn May by Alan Moorehead. The original MI5 files on the Fuchs case can be seen at The National Archives.