A bunch of events rapidly happening that were set in motion with the arrest of the British physicist called Klaus Funchs in Great Britain, 1950s, would culminate in the Rosenbergs being convicted, eventually, of espionage.
The authorities from Britain, associated with the FBI from the US, found enough evidence to be certain that Funchs, who had worked on the developing of the atomic bomb in both countries during World War II, passed top-secret information to the Soviets.
Fuchs ended up confessing that American Harry Gold was the one serving as a courier for the Soviet agents to whom Fuchs passed along information. When Gold was captured, the investigation reached Greenlass, who pointed to his sister and brother-in-law: Ethel and Julius Rosenberg.