In July 1946, Fenner was charged as a witness for the Nuremberg war crimes tribunal and in August transferred to the "Haus Alaska", a cover name for HQ 7707 European Command Intelligence Center Camp King, the US Army's interrogation centre, Oberursel (near Frankfurt), and interned with other high-ranking Germans. Fenner was intensively interrogated by the Army Security Agency (ASA) and wrote a number of reports about his life and work, including an autobiographical essay "The Career of William Fenner"—its English translation bearing the words TOP SECRET being filed in TICOM Archive: DF-187. These documents were made publicly available in 2008.