One of the rare applications for such elements in the 1970s was Boeing's supersonic transport development project, suggesting the possibility that Cooper was a Boeing employee. Other possible sources of the material included plants that manufactured cathode ray tubes, such as the Portland firms Teledyne and Tektronix. Over the 45-year span of its active investigation, the FBI periodically made public some of its working hypotheses and tentative conclusions, drawn from witness testimony and the scarce physical evidence. Cooper appeared to be familiar with the Seattle area and may have been an Air Force veteran, based on testimony that he recognized the city of Tacoma from the air as the jet circled Puget Sound, and his accurate comment to Mucklow that McChord AFB was approximately 20 minutes' driving time from the Seattle-Tacoma Airport—a detail most civilians would not know, or comment upon. His financial situation was very likely desperate. According to the FBI's retired chief investigator, Ralph Himmelsbach, extortionists and other criminals who steal large amounts of money nearly always do so because they need it urgently; otherwise, the crime is not worth the considerable risk. Alternatively, Cooper may have been "a thrill seeker" who made the jump "just to prove it could be done." 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 may have 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. Evidence suggested that Cooper was knowledgeable about flying technique, aircraft, and the terrain. He demanded four parachutes to force the assumption that he might compel one or more hostages to jump with him, thus ensuring he would not be deliberately supplied with sabotaged equipment. He chose a 727-100 aircraft because it was ideal for a bail-out escape, due to not only its aft airstair but also the high, aftward placement of all three engines, which allowed a reasonably safe jump despite the proximity of the engine exhaust.