The Oppenheimer security hearing was a 1954 proceeding by the United States Atomic Energy Commission (AEC) that explored the background, actions, and associations of J. Robert Oppenheimer, the American scientist who had headed the Los Alamos Laboratory during World War II, where he played a key part in the Manhattan Project that developed the atomic bomb. The hearing resulted in Oppenheimer's Q clearance being revoked. This marked the end of his formal relationship with the government of the United States, and generated considerable controversy regarding whether the treatment of Oppenheimer was fair, or whether it was an expression of anti-Communist McCarthyism.