In 1942, Biddle became involved in a case in which a military tribunal appointed by Roosevelt tried eight captured Nazi agents for espionage and for planning sabotage in the United States as part of the German Operation Pastorius. Lieutenant Colonel Kenneth Royall challenged Roosevelt's decision to prosecute the Germans in military tribunals by citing "Ex parte Milligan" (1866), a case in which the US Supreme Court ruled that the federal government could not establish military tribunals to try civilians in areas that civilian courts were functioning, even during wartime. Biddle responded that the Germans were not entitled to have access to civilian courts because of their status as unlawful combatants. The US Supreme Court upheld that decision in "Ex parte Quirin" (1942) by ruling that the military commission that was set up to try the Germans was lawful. On August 3, 1942, all eight were found guilty and sentenced to death. Five days later, six of the eight were executed in the electric chair on the third floor of the District of Columbia jail. The other two were given prison terms since they had willingly turned their comrades over to the FBI. In 1948, both men were released from prison and returned to Germany.