In January 1950, the German-born physicist Klaus Fuchs, working for the British mission in the Manhattan Project, was arrested for passing key documents to the Soviets throughout WWII. Fuchs identified his courier as the American Harry Gold. Gold, arrested in May 1950, then identified David Greenglass as an additional source.
In June 1950, under arrest by the FBI, Greenglass confessed to his courier role and also claimed his sister Ethel’s husband Julius Rosenberg had passed secrets to the Soviets.
Julius Rosenberg was arrested on suspicion of espionage in June 1950; his wife Ethel was arrested in August 1950. Another accused conspirator, Morton Sobell, fled but was arrested by U.S. forces in Mexico.
In March 1951, the Rosenbergs were convicted of espionage and sentenced to death. After a stay of execution and a world-wide campaign for clemency, the Rosenbergs were executed in June 1953.