Manhattan Project
Maintaining security was difficult because so many people worked on the project.  A special Counter Intelligence Corps detachment handled the project's security issues. By 1943, it was clear that the Soviet Union was trying to penetrate the project. Lieutenant Colonel Boris T. Pash, the head of the Counter Intelligence Branch of the Western Defense Command, investigated suspected Soviet espionage at the Radiation Laboratory in Berkeley. Oppenheimer informed Pash that he had been approached by a fellow professor at Berkeley, Haakon Chevalier, about passing information to the Soviet Union.