Project Blue Book was the Air Force name for a project that investigated UFO reports between 1947 and 1969. On December 17, 1969, the project was terminated, and all related documentation was turned over to the National Archives and Record Service.
Over the course of the investigation, 12,618 sightings were reported. 701 remain unidentified.
Now, the original Project Blue Book stated that they terminated because they found no threat to national security, no evidence that "unidentified" sightings represent technological advances beyond the then-known human scope, and no evidence that the sightings were extraterrestrial vehicles.
However. As with any government project, it doesn't seem to be all above board.
The National Archives received numerous inquiries concerning documents identified as "MJ12" and "Briefing Document: Operation Majestic 12." These searches were all negative with the exception of a "Memorandum for General Twining, from Robert Cutler, Special Assistant to the President, Subject: "NCS/MJ-12 Special Studies Project" dated July 14, 1954. The memorandum refers to a briefing to take place on July 16. It does not identify MJ-12 or the purpose of the briefing, and there is no other evidence.
On September 12, 1989, 10 years later, a new Project Blue Book apparently attempted to continue from the original. Someone contacted Mr. William S Sessions, Director of the FBI. I was able to find this letter in the FBI vault.
It shouldn't be a surprise that Mr. Sessions was of no help, and I'm not sure where the New Project Blue Book ended up going, if anywhere. This was the reply:
Of course the FBI was of no help. We civilians have to do everything ourselves.