Even before the war, a large Nazi spy ring was found operating in the United States. The "Duquesne Spy Ring" is still the largest espionage case in United States history that ended in convictions. The 33 German agents who formed the Duquesne spy 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 man opened a restaurant and used his position to get information from his customers; another worked at an airline so he could report Allied ships crossing the Atlantic Ocean; others in the ring worked as deliverymen so they could deliver secret messages alongside normal messages. The ring was led by Captain Fritz Joubert Duquesne, a South African Boer who spied for Germany in both World Wars and is best known as ""The man who killed Kitchener"" after he was awarded the Iron Cross for his key role in the sabotage and sinking of in 1916. William G. Sebold, a double agent for the United States, was a major factor in the FBI's successful resolution of this case. For nearly two years, Sebold ran a secret radio station in New York for the ring. Sebold provided the FBI with information on what Germany was sending to its spies in the United States while allowing the FBI to control the information that was being transmitted to Germany. On June 29, 1941, six months before the U.S. declared war, the FBI acted. All 33 spies were arrested, found or pled guilty, and sentenced to serve a total of over 300 years in prison.