The post-War period, in which Rosenberg lost his job, was typified by an unprecedented build up of Cold War intelligence agencies in the United States, United Kingdom and Soviet Union, and there was a great deal of political capital to be gained from knowing anything that the other side did not.
The first link in the chain of evidence, which led directly to Julius and Ethel Rosenberg, was an encoded KGB report on the Manhattan Project, America’s codename for the project to build an atomic bomb, which was intercepted by the FBI. Its contents led both the FBI and MI5 to suspect that a naturalised British scientist, Klaus Fuchs, could have been feeding details of the Manhattan Project to communist agents. An MI5 interrogation of Fuchs, then living in the UK, led, in turn, to the identification of his courier contact, known to him only as ‘Raymond’.
Whilst these complex investigations were underway, the threat of Communism was crystallised, in the first detonation of an atomic device by the Soviets, in August 1949. This made US intelligence services even more determined to trace the source of the leaking of the ‘Manhattan Project’ documents.
‘Raymond’ was eventually confirmed as a Jewish chemist, named Harry Gold who, in turn, gave up his own contact within the communist spy ring; a Los Alamos-based US soldier, whose identity was confirmed by investigators as David Greenglass, Ethel Rosenberg’s brother.