William G . Sebold William G . Sebold ( " Wilhelm Georg Debrowski " ; 10 March 1899 in Mülheim , Germany – February 1970 in Walnut Creek , California ) was a German spy in the United States during World War II , who became a double agent for the FBI . Early life . Sebold served in the German army during World War I . After leaving Germany in 1921 , he worked in industrial and aircraft plants throughout the United States and South America . On 10 February 1936 , he became a naturalized citizen of the United States . He returned to Germany in February 1939 to visit his mother in Mülheim . Upon his arrival in Hamburg , Germany , he was approached by a Gestapo agent who said that Sebold would be contacted in the near future . Sebold proceeded to Mülheim where he obtained employment . Espionage . In September 1939 , a Dr . Gassner visited Sebold in Mülheim and interrogated him regarding military planes and equipment in the United States . He also asked Sebold to return to the United States as an espionage agent for Germany . After additional visits by Dr . Gassner and a " Dr . Renken " ( later identified as Major Nickolaus Ritter of the Abwehr ) , Sebold agreed to spy for Germany , being blackmailed with information that he had omitted spending some time in a German jail when applying for American citizenship . Ritter was the Abwehr officer in charge of espionage against the United States and Britain . Sebold received final instructions from Ritter , including the use of codes and microphotographs . Sebold was then assigned the alias " Harry Sawyer " , the code name TRAMP , and Abwehr number A . 3549 . Before leaving Germany , Sebold went to the U . S . Consulate in Cologne , Germany . He told the consular officials about his future role as a German spy . He also expressed his wish to cooperate with the Federal Bureau of Investigation ( FBI ) in the United States . The U . S . government agreed . Sebold sailed from Genoa , Italy , and arrived in New York City on 8 February 1940 . There , Sebold ( with secret help from the FBI ) set himself up as a consulting engineer , with an office on 42nd Street in Manhattan . The FBI , forewarned of Sebold ' s arrival , set up Sebold ' s office so that they could record all conversations there , and even observe and film any meetings taking place ( via a one - way mirror ) . Duquesne Spy Ring . Sebold was instructed by the Abwehr to contact Fritz Joubert Duquesne , code - named DUNN , a German spy in New York . Duquesne had been a spy for Germany since World War I ; before that , he had been a Boer spy in the Second Boer War . At their first meeting , Duquesne , who was extremely worried about bugs and microphones in Sebold ' s office , gave Sebold a note suggesting that they should talk elsewhere . After relocating to an Automat , the two men exchanged information about members of the German espionage system with whom they had been in contact . In May 1940 , FBI agents on Long Island set up a shortwave radio station , and established contact with the Abwehr ' s radio station in Germany , posing as part of Sebold ' s spy ring . For 16 months this radio station was a main channel of communication between German spies in New York City and the Abwehr . During this time , the FBI ' s radio station transmitted over 300 messages to Germany , and received 200 messages from Germany . Through Sebold , the U . S . identified dozens of German agents in the United States , Mexico and South America . In June 1941 , all of Sebold ' s activities were stopped when the FBI arrested 33 German agents detected as a result of his activities . Of those arrested on the charge of espionage , 19 pleaded guilty . The 14 men who pled not guilty were tried in Federal District Court , Brooklyn , New York , on 3 September 1941 ; and they were all found guilty by the jury on 13 December 1941 . The thirty - three members of the Duquesne Spy Ring were sentenced to serve a total of over 300 years in prison . Duquesne himself was sentenced to 18 years in prison . The Duquesne Spy Ring remains the largest espionage case in U . S . history that ended in convictions . Sebold ' s success as a counterespionage agent is demonstrated by the successful prosecution of these 33 German agents . However , once the trial ended , Sebold disappeared . He is said to have entered a government witness protection program and assumed another identity . One of the earliest books to detail Sebold ' s career as a double agent was Alan Hynd ' s 1943 " " . Peter Duffy , author of " The Bielski Brothers " and " The Killing of Major Denis Mahon : A Mystery of Old Ireland " , is writing a book about Sebold for Scribner , a division of Simon and Schuster .