Agents theorized that Cooper took his alias from a popular Belgian comic book series of the 1970s featuring the fictional hero Dan Cooper, a Royal Canadian Air Force test pilot who took part in numerous heroic adventures, including parachuting. (One cover from the series, reproduced on the FBI website, depicts test pilot Cooper skydiving in full paratrooper regalia.) Because the Dan Cooper comics were never translated into English, nor imported to the U.S., they speculated that he had encountered them during a tour of duty in Europe. The Cooper Research Team suggested the alternative possibility that Cooper was Canadian, and found the comics in Canada, where they were also sold. They noted his specific demand for "negotiable American currency", a phrase seldom if ever used by American citizens; since witnesses stated that Cooper had no distinguishable accent, Canada would be his most likely country of origin if he were not a U.S. citizen.