Project Blue Book (1952-1969) was the Air Force's official UFO investigation program, but its lead scientist Dr. J. Allen Hynek later revealed it was primarily a public relations exercise designed to explain away sightings. The 1953 Robertson Panel, declassified in 1975, explicitly recommended that the government debunk UFO sightings to reduce public interest. Project Sign, Blue Book's predecessor, had actually recommended the extraterrestrial hypothesis in its 1948 'Estimate of the Situation,' but this was rejected and the document ordered destroyed by Air Force Chief of Staff General Vandenberg. Hynek became a prominent UFO researcher after leaving the project.