The case has spawned legions of "Cooperites" - self-appointed citizen sleuths and investigators who have called the FBI with tips and various theories about Cooper's real identity.
It all began Nov. 24, 1971, when a man who gave the name "Dan Cooper" bought a ticket on a Northwest Orient flight from Portland, Ore. to Seattle. As the plane took off, Cooper gave an attendant a note saying he possessed a bomb and then demanded $200,000. After the plane landed in Seattle, the passengers got off safely and the cash was brought aboard, but the crew stayed.
Cooper ordered the flight to head to Mexico, but as the jet flew over a forest in Washington state, the plane's back staircase opened and he jumped out with the money strapped to his body. He was never found. Law enforcement authorities had an artist draw up a now-famous portrait, known as the "Bing Crosby" sketch, showing a man in dark horn-rimmed sunglasses and suit.
Numerous suspects made headlines - more than a 1,000 were looked at - but none were ever charged with the crime. The bureau has also never ruled out that the hijacker could have been killed upon landing in the forest. The biggest lead came in 1980 when a family happened upon $5,800 in cash whose serial numbers matched the money that had been given to Cooper.
This summer, there was a burst of Cooper news. The History channel aired a documentary about a group of investigators who believed they had found their man: Robert "Bob" W. Rackstraw, a Vietnam War veteran and explosives expert who has over the years teased the public with denials and non-denials that he is Cooper. (He now insists that he is not.) The other development: The FBI officially closed the case.
Gray said his documents are voluminous. But they don't delve into the bureau's investigations of specific suspects.
"This is the real stuff. These are internal reports and actual investigative documents that the bureau used to investigate the case," Gray said. "There's so many files and so much information I thought it would be helpful to have all this out there so people can find things I missed."