Donald Keyhoe was an American Marine Corps naval aviator. He became a UFO researcher. In 1956, he cofounded the National Investigations Committee on Aerial Phenomena (NICAP). Keyhoe became director of NICAP in early 1957. He was one of the first and most forceful figures to press for Congressional hearings into the question of the UFO's.
Keyhoe had many contacts within the military and the Pentagon. His article "Flying Saucers Are Real" appeared in the January 1950 issue of True (a popular American magazine) and caused a sensation. Later on, he expanded the article into a book. In the final chapters of his book, Keyhoe studied over two hundred secret Air Force files released to him.
Keyhoe posited what the Extraterrestrial Hypothesis (ETH), that UFOs are spaceships of interplanetary origin. Both Keyhoe and the Air Force arrived at this conclusion by elimination. Some of the reported sightings could not be explained away as misidentifications or hoaxes. Neither the American military nor any of its allies or enemies possessed the aeronautical technology to produce aeroforms with the flight characteristics of the disks, nor did it make sense that the disks were experimental aircrafts.
Keyhoe wrote several more books about UFOs. Flying Saucers from Outer Space (1953) is perhaps the most impressive, being largely based on interviews and official reports by the Air Force. Albert Chop, the Air Force's press secretary in the Pentagon, characterized Keyhoe as a "responsible, accurate reporter" and expressed approval for Keyhoe's arguments in favor of the extraterrestrial hypothesis. This strengthened the popular belief that the Air Force's mixed messages about UFOs were due to a cover up.
Following a wave of UFO reports in 1966, NICAP was among those who called for an independent scientific investigation of UFOs. The Condon Committee was formed at the University of Colorado with this goal in mind, but it quickly became mired in controversy. Keyhoe publicized the so-called "Trick Memo", an embarrassing memorandum written by the Condon Committee coordinator which suggested that the objective and neutral Committee had determined to pursue a debunking operation well before even beginning their studies.
Several of Keyhoe's books are now in the public domain and are freely available online, such as: