In fact Stalin had long been aware of the program, despite the Manhattan Project having a secret classification so high that, even as Vice President, Truman did not know about it or the development of the weapons (Truman was not informed until shortly after he became president). A ring of spies operating within the Manhattan Project, (including Klaus Fuchs and Theodore Hall) had kept Stalin well informed of American progress. They provided the Soviets with detailed designs of the implosion bomb and the hydrogen bomb. Fuchs' arrest in 1950 led to the arrests of many other suspected Russian spies, including Harry Gold, David Greenglass, and Ethel and Julius Rosenberg.