Stoll, with the help of local authorities, traced the call to a Tymnet switch in Oakland, California. Because the call came from Oakland rather than Berkeley, it was obvious that the hacker was not working local to the university. Tymnet officials helped LBL trace the various calls, even though the hacker attempted to conceal their origin. Enlisting the aid of AT&T and the Federal Bureau of Investigation (FBI), Stoll eventually determined that the calls were being "piggybacked" across the United States, but originating from Hanover, Germany.