Gerald Haines, a historian at the National Reconnaissance Office, said that when called upon to explain alleged UFO sightings during the Cold War years, the US Air Force frequently provided explanations that were untrue in order to deflect attention from their planes.
"Over half of all UFO reports from the late 1950s through the 1960s were accounted for by manned reconnaissance flights (namely the U-2) over the United States," he wrote in the spring issue of Studies of Intelligence, an unclassified CIA journal.
Concern about the public finding out about the secret spy planes "led the Air Force to make misleading and deceptive statements to the public in order to allay public fears and to protect an extraordinarily sensitive national security project," Mr Haines wrote.
"While perhaps justified, this deception added fuel to the later conspiracy theories and the cover-up controversy ..." regarding the existence of UFOs, he added.
Mr Haines based his article on a review of CIA documents from the late 1940s to 1990.
He described how the Air Force sought to deflect attention from the development of its high-altitude experimental aircraft, the U-2 and the SR-71.
The early U-2s were silver and reflected the sun's rays, especially at sunrise and sunset, and often appeared as fiery objects to people below, Mr Haines said. The U-2s were later painted black. Air Force investigators, "aware of the secret U-2 flights, tried to explain away such sightings by linking them to natural phenomena such as ice crystals and temperature inversions," Mr Haines wrote.
By 1956 the Air Force internally had clear explanations for 96 per cent of all UFO sightings, Mr Haines wrote, referring to the experimental aircraft. "They were careful, however, not to reveal the true cause of the sighting to the public."
He also said that the CIA, during the height of the Cold War, hid its involvement in studies into UFO sightings because the agency was concerned that if word came out it would lead to a national hysteria that could be exploited by the Soviet Union.
The director of space policy at the Washington-based Federation of American Scientists, John Pike, said the study raises questions about other possible government cover-ups involving unidentified flying objects.
"The flying-saucer community is definitely on to something," in accusing the military of hiding something, Mr Pike told the New York Times, which reported on the study in yesterday's edition.Reuse content