One of the Longest and Most Exhaustive Investigations
The FBI was forced to start what would eventually become known as “one of the longest and most exhaustive investigations” in its history. The Bureau called it the NORJAK (also known as the Northwest Hijacking). Initially, the FBI had believed that Cooper knew both the planes as well as the area very well, and they were pretty much sure that he served in the military, maybe even as a paratrooper.
Source: In Public Safety
But the FBI later changed their minds in the end. They decided that he was not an experienced skydiver. Why? Well, because not only was the jump just too dangerous, he also failed to notice that his reserve parachute was actually sewn shut for the purposes of military training. So there was that…