Project Blue Book was one of several U.S. Air Force investigations into UFOs. Their documentation was transferred to the National Archives. The materials include home movies and are highlighted in the NARA blog The Unwritten Record. The FBI also collected information on this program which is available in the FBI Vault. Project Bluebook- 1947-1969.
In 1966 testimony was given by the Secretary of the Air Force, Harold Brown. Regarding UFO he stated: "the past 18 years of investigating unidentified flying objects have not identified any threat to our national security, or evidence of extraterrestrial vehicles” (Unidentified flying objects : hearing by Committee on Armed Forces of the House of Representatives, Eighty-ninth Congress, second session. April 5, 1966, United States. Congress. House. Committee on Armed Services. 5592).
In the 1950s the U.S. Air Force was working on developing a saucer-shaped aircraft (image above). There is speculation that Project 1794 is the origin of the flying saucer sightings. Project 1794, Final Development Summary Report (d.1956)
In 1966 the University of Colorado was chosen by the Air Force Office of Scientific
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esearch to manage a program to study UFOs. This was known as the University of Colorado UFO Project (Condon Committee) 1966-1968. Their 1968 report: Final Report of the Scientific Study of Unidentified Flying Objects, Conducted by the University of Colorado Under Contract to the United States Air Force, was reviewed by the National Academy of Science which issued Hypotheses of Extraterrestrial Origin.
Image: The areoshell of a NASA Voyager-Mars space probe