"During World War II, Klaus Fuchs worked on the Manhattan Project...and later worked on British nuclear projects." In 1950, the FBI decoded messages that revealed Fuchs as a Soviet spy. "He admitted spying for the Russians since 1942 and passing on details of British and American nuclear technology. The arrest of Fuchs led authorities to several other individuals involved in a spy ring, culminating with the arrest of Julis and Ethel Rosenberg and their subsequent execution."
Klaus Fuchs was a German-British theoretical physicist and atomic spy who in 1950 was convicted of supplying information from the American, British and Canadian atomic bomb research (the Manhattan Project) to the USSR during and shortly after World War II. While at the Los Alamos National Laboratory, Fuchs was responsible for many significant theoretical calculations relating to the first fission weapons and later, the early models of the hydrogen bomb, the first fusion weapon.
Espionage information procured by Klaus Fuchs and Theodore Hall, and to a lesser extent by David Greenglass, led to the first Soviet device, "RDS–1", closely resembling Fat Man, even in its external shape.