During World War II, three known persons engaged in espionage activities at Los Alamos: Klaus Fuchs, Theodore Hall, and David Greenglass. It is possible that a fourth spy, code-named Perseus, also worked at Los Alamos during this time.
Klaus Fuchs came to Los Alamos with the British Mission. He was a German communist who had immigrated to the United Kingdom to escape Nazi persecution. Fuchs began spying for the Soviet Union while living in the U.K. At Los Alamos, Fuchs worked on implosion problems and delivered sketches of Fat Man to his Soviet-run courier, Harry Gold. After the war, Fuchs returned to the U.K. where he worked at Harwell, the British nuclear research laboratory. Using information gathered from decrypted Soviet intelligence cables, the FBI and British intelligence began questioning Fuchs in 1949. He ultimately confessed and was convicted of espionage in a two-day trial. He spent 9.5 years in Wormwood Scrubbs prison and moved to East Germany after his release. He died in East Berlin in 1988.
Theodore Hall graduated from Harvard at the age of 18 and was recruited to work at Los Alamos because of his technical training. Hall worked primarily on instrumentation for the implosion program. During a vacation, Hall walked into the Soviet embassy in New York and volunteered to serve as a spy, believing that the Soviet Union needed to be informed of the United States' work to develop a fission bomb. After the war, Hall moved to Chicago to work on his doctorate. The FBI's Venona program transcripts identified him as a possible spy, but for unknown reasons, he was not arrested. Hall immigrated to Great Britain in the early 1960's, living there until his death in 1999.
David Greenglass, a machinist, came to Los Alamos as part of the Army's Special Engineer Detachment. Upon learning of Greenglass's assignment to Los Alamos, his sister Ethel Rosenberg and her husband Julius recruited him to join their espionage ring. According to court records, Greenglass provided some sketches of Fat Man components to the Rosenbergs. When the Rosenberg spy ring was uncovered, Greenglass agreed to a plea bargain and provided key evidence that led to the conviction and execution of Julius and Ethel Rosenberg.
In the early 1990s, evidence from the KGB archives suggested the possibility that a fourth spy was active at Los Alamos during World War II. Known only by the codename Perseus, s/he remains an enigma today. There are only fleeting references to Perseus in the Venona files. The KGB archives were closed before extensive research could be done. Opinion is evenly split on whether or not Perseus was real.