The United States Air Force began investigations of UFOs back in 1947, which continued until 1969. For decades, the U.S. government denied it had investigated UFOs.
However, about 20 government employees came forward in 2001 and told ABC News that the U.S. should come clean. It was also reported that President George Bush refused to release such information in the 1970s when he was head of the CIA.
Eventually much, but not all, of the U.S. government's files on UFOs were declassified and released. However, the thousands of files were only accessible at the National Archives in Washington D.C.
The files were uploaded by John Greenewald who began submitting Freedom of Information Act (FOIA) requests for UFOs back when he was a teen.
Project Blue Book was the name of the third investigation of UFOs by the U.S. government from 1951 to 1969. Earlier investigations included Project Sign, from 1947 to 1948, and Project Grudge, which went from 1949 to 1951.
"There is plenty of work for amateur investigators to try to come up with explanations they never had time to consider or research," Nigel Watson, author of the "UFO investigations Manual," told the Daily Mail.