Rosenberg provided thousands of classified reports from Emerson Radio, including a complete proximity fuse. Under Feklisov's supervision, Rosenberg recruited sympathetic individuals into NKVD service, including Joel Barr, Alfred Sarant, William Perl, and Morton Sobell, also an engineer. Perl supplied Feklisov, under Rosenberg's direction, with thousands of documents from the National Advisory Committee for Aeronautics, including a complete set of design and production drawings for Lockheed's P-80 Shooting Star, the first U.S. operational jet fighter. Feklisov learned through Rosenberg that Ethel's brother David Greenglass was working on the top-secret Manhattan Project at the Los Alamos National Laboratory; he directed Julius to recruit Greenglass.

In February 1944, Rosenberg succeeded in recruiting a second source of Manhattan Project information, engineer Russell McNutt, who worked on designs for the plants at Oak Ridge National Laboratory. For this success Rosenberg received a $100 bonus. McNutt's employment provided access to secrets about processes for manufacturing weapons-grade uranium.

The USSR and the U.S. were allies during World War II, but the Americans did not share information with, or seek assistance from, the Soviet Union regarding the Manhattan Project. The West was shocked by the speed with which the Soviets were able to stage their first nuclear test, "Joe 1", on August 29, 1949.However, the head official of the Soviet nuclear project, Lavrentiy Beria, used foreign intelligence, which he did not trust by default, only as a third-party check, rather than giving it directly to the design teams, who he did not clear to know about the espionage efforts, and the development was indigenous; considering that the pace of the Soviet program was set primarily by the amount of uranium that it could procure, it is difficult for scholars to judge accurately how much time was saved, if any.