The FBI had been tangling Axis spies throughout the mid-to-late Thirties, but with the December 8, 1941, declaration of war the FBI was emboldened with far greater powers. This explains why Director Hoover exclaimed "that his agency had just completed the busiest year in its history."
"In his annual report ended June 30, he disclosed the FBI agents had produced evidence which brought 1,471 convictions for espionage, sabotage, Selective Service Act violations and other un-American activities."
An an example of the FBI's productivity can be read here
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