Throughout the development of the atomic bomb, Oppenheimer was under investigation by both the FBI and the Manhattan Project's internal security arm for his past left-wing associations. He was followed by Army security agents during a trip to California in June 1943 to visit Tatlock, who was suffering from depression. Oppenheimer spent the night in her apartment. Tatlock killed herself on January 4, 1944, leaving Oppenheimer deeply grieved.