The largest espionage case in the US was the Duquesne spy ring. Led by Fritz Duquesne who despised the British and became a spy for Germany in both World Wars I and II, the ring was caught and prosecuted by the FBI in 1941. Interestingly they were found guilty on December 13th, 1941. I would image that having a jury deliberate on whether you are a traitor a few days after Pearl Harbor was not what they would have hoped for. In January the group was sentenced to a total of 300 years in prison.
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