June 6 was a cloudy, drizzling day. Two agents from the Federal Bureau of Investigation came to Tsien's office. Tsien now occupied Kármán's old room on the second floor of the Guggenheim Building. It was a large room with Chinese journals on the tables. Behind his desk were blackboards covered with mathematical equations. A big window opened to a view of the courtyard. What the FBI wanted to know was simple: Was Tsien, or had Tsien ever been, a member of the Communist Party?
The FBI claimed that several people Tsien had befriended at Caltech in the 1930s were Communists. The social gatherings held at Sidney Weinbaum's home were, they said, in reality meetings of Professional Unit 122 of the Pasadena Communist Party. Tsien's name had somehow appeared on 1938 membership lists linked with the alias "John Decker." The agent interrogated Tsien to learn more about his relationships with the party and with Weinbaum.
Tsien repudiated all the charges. He denied that he had ever been a Communist. In fact, he insisted he was philosophically opposed to the idea of Communism. Russian Communism, in his opinion, was nothing more than a totalitarian form of government, and relative to democratic or free government it