DENVER (AP) — A chance encounter at a restaurant in a Denver suburb led to the arrest of an Ethiopian immigrant who authorities say tortured political prisoners decades ago in his home country, according to testimony from a Department of Homeland Security official Tuesday.

The man authorities identify as Kefelegn Alemu Worku (kah-FEH’-lun ah-LEE’-moo WER’-koo) pleaded not guilty to U.S. immigration charges and denied that he is a former guard at a prison know for atrocities and human rights abuses.