On 28 June 1941, following a two-year investigation, the FBI arrested Col. Duquesne and 32 Nazi spies on charges of relaying secret information on U.S. weaponry and shipping movements to Germany. On 2 January 1942, less than a month after the U.S. was attacked by Japan at Pearl Harbor and Germany declared war on the United States, the 33 members of the Duquesne Spy Ring were sentenced to serve a total of more than 300 years in prison. They were found guilty in what historian Peter Duffy said in 2014 is "still to this day the largest espionage case in the history of the United States". One German spymaster later commented that the ring's roundup delivered 'the death blow' to their espionage efforts in the United States. J. Edgar Hoover called his FBI swoop on Duquesne's ring the greatest spy roundup in U.S. history. In a 1942 memo to his superiors, Canaris reported on the importance of several of his captured spies by noting their valued contributions, and he writes that Duquesne "delivered valuable reports and important technical material in the original, including U.S. gas masks, radio-controlled apparatus, leak proof fuel tanks, television instruments, small bombs for airplanes versus airplanes, air separator, and propeller-driving mechanisms. Items delivered were labeled 'valuable', and several 'good' and 'very good.