Many of the bit roles in this film were played by real FBI agents, and this was their only film.
The film is loosely based on the case of Duquesne Spy Ring headed by Frederick Joubert Duquesne and the work of real life double agent William G. Sebold.
The man who is killed by the car near the beginning of the film is based on a real life incident. He was identified as Julio Lopez Lido but was in actuality Capt. Ulrich von der Osten, a Nazi army officer in the Abwehr. He was hit by a car on March 18, 1941 and his body went unclaimed for a time. The man who ran from the scene was actually Kurt Frederick Ludwig, known as Joseph K, and eventually sentenced to Alcatraz Island. He was deported in 1953. The cab driver who hit Lido was a man named Sam Lichtman.
The story is also based on aspect of the Ludwig Spy case, involving German agent Kurt Frederick Ludwig, known as Joseph K.