	By the early 1940s, US Army Intelligence was already aware of allegations that Tsien was a Communist, but his security clearance was not suspended. However, on June 6, 1950 his security clearance was revoked and Tsien was questioned by the FBI. Two weeks later Tsien announced that he would be resigning from Caltech and returning to China, which by then was effectively governed by the Communist Party of China led by Mao Zedong.