Today, if you ask the Air Force about UFOs, it will cite its own 22-year study called Project Blue Book, which said there is no evidence that they are extraterrestrial vehicles and there is no evidence that they represent technology beyond our own.
Blue Book, based at Wright-Patterson Air Force Base in Ohio, investigated hundreds of UFO reports yearly throughout the 1950s and 1960s.
But the truth is Blue Book never became a serious, full-scale, scientific inquiry. The main purpose of the Air Force's UFO office was public relations, says Robert Goldberg, author of "Enemies Within: The Culture of Conspiracy in Modern America."
"That mission was denounce the UFOs, dismiss the UFOs, debunk the UFOs and anybody who believes in them -- just come up with answers and get this UFO thing out of the newspapers," he told ABC News.
Blue Book was far from a massive institute with a staff of white-coated lab technicians, said UFO researcher Mark Rodeghier. "There was a guy at a desk and a secretary and a private or someone there typing stuff. It was a very, very small project," he said.
Blue Book may have done some investigating, but it was overwhelmed by the volume of reports that were coming in.
Col. Robert Friend, the project's director from 1958 to 1963, told ABC News: "We wanted to explain as many sightings as possible, but we recognized that the amount of resources that would have been necessary in order to do this would have been far beyond those that we were ready to commit at the time."
He also recognized Project Blue Book's real purpose: "What they wanted to try to do was, I think, to re-educate the public regarding UFOs, to take away the aura of mystery."
And the best way to keep UFOs out of the newspapers -- and therefore, out of the public mind -- was to say repeatedly that they were nothing more than weather balloons or rare atmospheric conditions, like a star on the horizon.
The man most often responsible for making these explanations was Blue Book's one civilian scientist, Ohio State University astronomer J. Allen Hynek. Between 1948 and 1969, he was the lead investigator on thousands of cases.
In interviews from that time, he insisted "there is no proof that I would consider valid scientific proof that we have been visited by spaceships."
Michael Swords, a professor of natural science at Western Michigan University and UFO researcher says Hynek's job "was to stretch his imagination to try to find explanations for every possible case he could, even if he knew it didn't make any sense."
In a 1965 interview with one witness, Hynek argued with a woman who said she saw a UFO, insisting it was actually a meteor.
She asked, "Don't you think it would be kind of unusual for a meteor to just fall across the road and hover over there a minute and then drop to the ground?"
Hynek replied: "The coming over wouldn't be bad. It's the hovering that would bother me."
Project Blue Book even dismissed a sighting by experienced military personnel on high alert during the middle of the Cold War.
On the night of Oct. 24, 1968, Mike O'Connor was dispatched to make a repair at a missile site at Minot Air Force Base in North Dakota.
En route, he says he saw a bright light "lift off the ground, and parallel us down the road, until we came to the missile site." When he got out of the truck, the light "just kind of hovered there," he said.