Upon his arrival in Hamburg, Germany, he was approached by a Gestapo agent who said that Sebold would be contacted in the near future due to the knowledge he obtained while working in United States aircraft factories.

[2] In September 1939, a Dr. Gassner visited Sebold in Mülheim and interrogated him regarding military planes and equipment in the United States.

Gassner and another man, a "Dr. Renken", told him that they would expose information that he had omitted from his U. S. citizenship application about serving time in a German jail unless he agreed to assist them.

He was then sent to a seven-week training program in Hamburg, Germany, where he learned to operate a clandestine shortwave radio, which he would set up when he returned to the United States.

[4] Ritter gave Sebold final instructions before he left for the United States, including shortwave radio codes and the use of microphotographs.

He told the Consul that he had been blackmailed into becoming a German spy but that he was a loyal American citizen and wanted to cooperate with the Federal Bureau of Investigation (FBI) in the United States.

This office provided a seemingly safe space where Nazi spies felt comfortable meeting with and discussing their plans with Sebold.

For example, the group's leader Fritz Joubert Duquesne was caught discussing how fires could be started at industrial plants to slow production, and showed photographs of blueprints for a new bomb being built in the United States.

In different footage, a spy explains his plan to bomb a building, going as far as bringing dynamite and detonation caps to Sebold's office.

Sebold was instructed by the Abwehr to contact Fritz Joubert Duquesne, code-named DUNN, a German spy in New York.

Duquesne, who was vehemently anti-British, submitted information dealing with national defense in America, the sailing of ships to British ports, and technology.

[2] On one occasion, Duquesne provided Sebold with photographs and specifications of a new type of bomb being produced in the United States.

As a result of the massive investigation, when the United States entered the war the FBI was confident that there was no major German espionage network hidden in U.S.

Additionally, letters from his family back in Germany explained that the Nazis still wanted to exact their revenge, leaving him in a constant state of fear.