From the time Adolf Hitler came into power in 1933, German spies were active in New York. As war began in Europe in 1939, a German-American was recruited by the Nazis to set up a radio transmitter and collect messages from spies to send back to Nazi spymasters. Peter Duffy tells us how this German-American, William G. Sebold, became the FBI’s first double agent, spearheading a covert mission to infiltrate New York’s Nazi underground in the days leading up to World War II—it was the most successful counterespionage operation in U.S. history. Duffy tells the story in his book, Double Agent: The First Hero of World War II and How the FBI Outwitted and Destroyed a Nazi Spy Ring. .