Since the hijacking, there have been over a 1,000 suspects, only a few actually viable.
The person who was most looked into was Richard Floyd McCoy, Jr. McCoy was responsible for what some say was a "copycat D.B. Cooper". He hijacked a plane in 1972, parachuting to safety. He had also kept passengers ransom for money, though he left behind far more evidence than Cooper had done the previous year.
McCoy had military experience and was an experienced skydiver. The FBI had suspected Cooper of having once been in the military.
Extra facts: Cooper had a parachute that wasn't very steerable, especially having been in the weather he jumped in. It was very windy and cold and rainy that night. There was a nearby river where it's believed he had jumped, with heavy rapids, also it's the river money was found by.
All the evidence that was left on the plane was Cooper's tie, 8 cigarettes he had smoked, a glass of bourbon and soda, and a second parachute that he did not use for the jump.