In 1950, a junior senator named Joseph McCarthy went on the offensive against communist subversion. Unlike many other investigators, McCarthy declared that he possessed concrete facts: a list containing the names of Soviet sympathizers holding high ranks in the US government. McCarthy went on to terrorize and abuse people from all walks of life for several years. The senator's investigation into high-ranking military personnel led to his final undoing and censure by the Senate. In retrospect, people believe that McCarthy never had a legitimate list of names and that his attacks were predominantly fabricated, leading to the popular conception of McCarthy as an amoral political opportunist.

What was likely McCarthy's most potent tool in his inquiries?