The Duquesne Spy Ring is the largest espionage case in the United States history that ended in convictions. A total of 33 members of a Nazi German espionage network headed by Frederick "Fritz" Joubert Duquesne were convicted after a lengthy investigation by the Federal Bureau of Investigation (FBI). Of those indicted, 19 pleaded guilty. The remaining 14 were brought to jury trial in Federal District Court, Brooklyn, New York, on September 3, 1941; all were found guilty on December 13, 1941. On January 2, 1942, the group members were sentenced to serve a total of over 300 years in prison.

The agents who formed the Duquesne Ring were placed in key jobs in the United States to get information that could be used in the event of war and to carry out acts of sabotage: one opened a restaurant and used his position to get information from his customers; another worked on an airline so that he could report Allied ships that were crossing the Atlantic Ocean; others worked as delivery people as a cover for carrying secret messages.

William G. Sebold, who had been blackmailed into becoming a spy for Germany, became a double agent and helped the FBI gather evidence. For nearly two years, the FBI ran a shortwave radio station in New York for the ring. They learned what information Germany was sending its spies in the United States and controlled what was sent to Germany. Sebold's success as a counterespionage agent was demonstrated by the successful prosecution of the German agents.

One German spymaster later commented the ring's roundup delivered "the death blow" to their espionage efforts in the United States. FBI director J. Edgar Hoover called his concerted FBI swoop on Duquesne's ring the greatest spy roundup in U.S. history.The 1945 film The House on 92nd Street was a thinly disguised version of the Duquesne Spy Ring saga of 1941.

The Federal Bureau of Investigation (FBI) is the domestic intelligence and security service of the United States and its principal federal law enforcement agency. Operating under the jurisdiction of the United States Department of Justice, the FBI is also a member of the U.S. Intelligence Community and reports to both the Attorney General and the Director of National Intelligence. A leading U.S. counter-terrorism, counterintelligence, and criminal investigative organization, the FBI has jurisdiction over violations of more than 200 categories of federal crimes.Although many of the FBI's functions are unique, its activities in support of national security are comparable to those of the British MI5, the New Zealand GCSB and the Russian FSB. Unlike the Central Intelligence Agency (CIA), which has no law enforcement authority and is focused on intelligence collection abroad, the FBI is primarily a domestic agency, maintaining 56 field offices in major cities throughout the United States, and more than 400 resident agencies in smaller cities and areas across the nation. At an FBI field office, a senior-level FBI officer concurrently serves as the representative of the Director of National Intelligence.Despite its domestic focus, the FBI also maintains a significant international footprint, operating 60 Legal Attache (LEGAT) offices and 15 sub-offices in U.S. embassies and consulates across the globe. These foreign offices exist primarily for the purpose of coordination with foreign security services and do not usually conduct unilateral operations in the host countries. The FBI can and does at times carry out secret activities overseas, just as the CIA has a limited domestic function; these activities generally require coordination across government agencies.

The FBI was established in 1908 as the Bureau of Investigation, the BOI or BI for short. Its name was changed to the Federal Bureau of Investigation (FBI) in 1935. The FBI headquarters is the J. Edgar Hoover Building, located in Washington, D.C.