Beginning in the 1940s and continuing into the 1970s, the bureau investigated cases of espionage against the United States and its allies. Eight Nazi agents who had planned sabotage operations against American targets were arrested, and six were executed (Ex parte Quirin) under their sentences. Also during this time, a joint US/UK code-breaking effort (the Venona project)—with which the FBI was heavily involved—broke Soviet diplomatic and intelligence communications codes, allowing the US and British governments to read Soviet communications. This effort confirmed the existence of Americans working in the United States for Soviet intelligence. Hoover was administering this project but failed to notify the Central Intelligence Agency (CIA) until 1952. Another notable case is the arrest of Soviet spy Rudolf Abel in 1957. The discovery of Soviet spies operating in the US allowed Hoover to pursue his longstanding obsession with the threat he perceived from the American Left, ranging from Communist Party of the United States of America (CPUSA) union organizers to American liberals.

When did the FBI begin investigating espionage?