From the FBI...
The night dragged on with no new leads being developed. Within a few hours, though, area residents woke up to news of the hijacking. The Salt Lake City Office of the FBI received a telephone call before noon from a concerned citizen who said an acquaintance had outlined a “foolproof” plan for hijacking an airplane. The acquaintance, Richard Floyd McCoy, Jr., was a Vietnam veteran, a helicopter pilot, and an avid and capable skydiver. McCoy, 29, was reportedly a member of the Utah Air National Guard and a Police Science Major at Brigham Young University. In addition, McCoy was generally known to be having serious financial problems.
Meanwhile, the FBI Laboratory had been busy with evidence already received. A handwriting expert compared the note found on the plane with McCoy’s writing on military service records and determined that McCoy had written the note. Fingerprint specialists discovered that a latent print on the “Mainliner Magazine” found in the seat next to the hijackers’s matched a print taken from McCoy during his service year.
The hijacker opened his luggage and covered the peephole between the cockpit and cabin. Observed by Second Officer Floyd Smith (fictitious name) through a slight space under the cockpit door, the hijacker quickly put on a jumpsuit, helmet, and parachute. Once he had shut off the cabin lights to better view the ground, the gunman demanded to be kept abreast of wind, ground, and air speeds; altimeter settings; and sky conditions.
I believe it was a police officer McCoy knew that gave him away...also, it appears his prints were found on the plane. I'm sure the FBI checked those against the prints found on 305. McCoy even covered the peephole on the cabin door leading to the cockpit. he brought his own chute & gear. McCoy also showed his experience of being a pilot by acting as the co-pilot with constant updates.
NONE, of the evidence from that book as been verified by anyone. McCoy's letters were typed except the one he forgot to retrieve from the stewardess that really did him in. one of Cooper's requests before takeoff was that the cabin light stay off.