Here's the gist of the FBI's theory:
In the fictional series, Royal Canadian Air Force test pilot Dan Cooper takes part in adventures in outer space and real events of that era. In one episode, published near the date of the hijacking, the cover illustration shows him parachuting.
Seattle Special Agent Larry Carr, who took over the Cooper case two years ago, believes it’s possible the hijacker took his name from the comic book (the enduring “D.B.” was actually the result of a media mistake). That’s important because the books were never translated into English, which means the hijacker likely spent time overseas. This fits with Carr’s theory that Cooper had been in the Air Force (see sidebar).
Special Agent Carr's theory, incidentally, is that maybe Cooper was one of the hundreds of thousands of U.S. military troops who spent time in Europe in the '60s and early '70s, and maybe he had some kind of duties that called for him to wear a parachute without being a trained parachutist - as the skyjacker D.B. Cooper evidently wasn't - and maybe he then got a job in the aviation industry in the Pacific Northwest, and then maybe he was laid off in 1971. That's a lot of maybes.
I can't help but speculate that Special Agent Larry Carr must not be a rising star in the FBI. How badly did he have to screw up to get assigned to the case of a 1971 skyjacking and robbery by an unknown suspect who almost certainly did not survive his skydive?
Comicanuck, an expert on French-Canadian comic books - yes, seriously - examines the comic book connection at length here: The Pursuit of Dan Cooper RCAF: FBI explores link between DB Cooper and comic book hero.
Comicanuck solves the case to my satisfaction by pointing out the uncanny physical resemblance between Special Agent Carr (below, on left) and Dan Cooper