Final answer:
The primary informant behind the identification of Alger Hiss as a Soviet collaborator was a Soviet spy who led to the exposure of the 'pumpkin papers'. Klaus Fuchs, another informant, confessed to leaking atomic bomb information, which resulted in a series of arrests, including that of the Rosenbergs.
Step-by-step explanation:
Lansing's primary informant was a Soviet spy who identified Alger Hiss, an official in the State Department, as a collaborator with the Soviets. The Soviet spy's revelation led to the discovery of the 'pumpkin papers', which were microfilmed documents allegedly typed by Hiss and passed to his Soviet contact. Despite the lack of concrete proof that Hiss produced the documents and due to the statute of limitations on espionage, Hiss was not tried for espionage but for perjury, for which he was eventually convicted. In a separate but related context, Klaus Fuchs, a British physicist, confessed to passing information on America's atomic bomb to the Soviets, which led to the arrests of the Rosenbergs in the U.S. Fuchs implicated Harry Gold as the go-between, who then implicated David Greenglass, and subsequently, Greenglass implicated his sister-in-law Ethel Rosenberg and her husband Julius Rosenberg.