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The FBI report details how “before America ever fired a shot” in World War 2 , they uncovered a massive spy ring. The work was carried out by a naturalised American named William Sebold, who turned against Nazis and worked with the US to bring the secret agents to justice. In total 33 spies were caught and collectively sentenced to more than 300 years in prison. In unearthed accounts, the FBI praise Mr Sebold for refusing to “give in to Nazi aggression and hatred” in one of its biggest cases.

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William Sebold moved to the US after the end of World War 1 – where he had served in the German army.

Looking for a different life, he became a US citizen after he immigrated in 1921.

But his loyalty to the US would be tested 18 years later when he returned to Germany for the first time and was approached by high ranking Nazis.

They wanted Mr Sebold to join their cause and become a spy.

There he would be expected to transmit messages back to Germany, conduct acts of sabotage and help other spies landing on US soil.

After being “persuaded” by the secret service, Mr Sebold was taken to Hamburg for espionage training – but not without speaking to the US consulate first.