On the afternoon of June 24, 1947, amateur aviator Kenneth Arnold was flying near Mt. Rainier, Washington, when he suddenly spotted nine unusual objects on the horizon. Arnold claimed the craft flitted from side to side and flipped in unison like “the tail of a Chinese kite,” and he estimated they were moving at around 1,700 miles per hour—far faster than any known aircraft. He initially assumed the physics-defying objects must be secret military vehicles, but he later admitted the incident was “as much a mystery to me as it is to everybody else.”

Arnold’s extraordinary story soon found its way into newspapers across the country, and reporters pounced on his description of the objects as moving “like a saucer if you skip it across water.” Within days, the term “flying saucer” was born.

Coupled with the famed July 1947 incident at Roswell, New Mexico, when the Air Force claimed a military weather balloon was mistaken for an alien spacecraft, Arnold’s encounter helped spark a wave of “flying saucer” sightings across the United States. The military brushed aside most of these “close encounters” as misidentifications or mere hokum, but a few reports came from air-traffic controllers and commercial pilots—people trained to search the skies with a discerning eye. The hysteria also dovetailed with the beginning of the Cold War, leading many to speculate that the mysterious sightings might be hostile Soviet aircraft.

Thus began official government investigations into the mysterious phenomena.

Following an official Air Force inquiry, Lt. General Nathan Twining fired off a memo in late-1947 describing the “flying disc” phenomenon as “something real and not visionary or fictitious.” He suggested the military launch an investigation into the source of the sightings.

By 1948, the Air Force had initiated “Project Sign,” the first of three military offices tasked with collecting and analyzing reports of what were termed “Unidentified Flying Objects.” Project Sign’s investigators quickly concluded that UFOs weren’t coming from behind the Iron Curtain—their flight characteristics simply didn’t match those of any manmade aircraft—but some on the team may have embraced the idea that UFOs were not of this world.

According to Air Force officer Edward Ruppelt and others who studied UFOs for the government, Project Sign produced a report in the summer of 1948 speculating that the sightings might be evidence of “interplanetary” or extraterrestrial craft. Air Force brass supposedly rejected and destroyed the document on the grounds that there was no hard evidence for its conclusions. To this day, no copies of the report have ever been recovered.

Project Sign was terminated in late 1948 and replaced by the short-lived Project Grudge, which was later succeeded in 1951 by the now-famous Project Blue Book. Based at Wright-Patterson Air Force Base near Dayton, Ohio, Blue Book served as the government’s main repository for sightings of unidentified aerial phenomena. Over the next 18 years, its tiny staff investigated thousands of reports and often went into the field to interview Americans who had experienced close encounters with all manner of flying saucers and discs, cigar-shaped rockets and dazzling nighttime lights.

The “Blue Book” era began with a bang. Projects Sign and Grudge had only averaged around 170 UFO reports each year, but 1952 brought an unprecedented 1,501 sightings.

Perhaps the most extraordinary of all came in July 1952, when a series of unusual blips suddenly lit up radar screens across Washington, D.C. Bewildered military personnel scrambled jets to intercept the bogies, but while their pilots reported seeing bright lights dancing through the night sky, they were unable to catch them.

In the wake of the sightings, the U.S. Air Force held a press conference in which Major General John Samford said the government would continue to investigate reports made by “credible observers of relatively incredible things.” Samford said the events in Washington may have been “temperature inversions”—layers of warm air that can cause radar aberrations—and he assured Americans that UFOs did not bear “any conceivable threat to the United States.”

Despite Samford’s claims, many in President Harry Truman’s administration were indeed concerned that UFOs were a safety hazard. Whether the sightings were real or just mass hysteria, reports from panicked citizens ran the risk of choking federal communications channels. Some in the CIA even believed the Soviets could stage a UFO incident to help screen an attack on the United States.

In January 1953, the CIA convened a group of experts under the direction of CalTech physicist H.P. Robertson to review the flying-saucer issue. This “Robertson Panel” concluded that most UFO sightings could be easily explained away as harmless optical illusions or weather phenomena. Still, the group suggested the government should take steps to debunk UFO events to help prevent a potential public uproar. In a move that would provide fuel for conspiracy theorists’ fires for years to come, they also suggested that the feds soothe the national consciousness by using mass media, celebrities and even the Walt Disney Company to ridicule and discredit UFOs.

With the help of civilian astronomer J. Allen Hynek, Project Blue Book’s investigators spent the next several years debunking UFO sightings as everything from hoaxes and misidentified aircraft to birds, weather balloons, astronomical phenomena and contrails. The team successfully cleared up thousands of cases, yet their explanations often seemed as unbelievable as the reports themselves. A 1966 UFO in Michigan was blamed on “swamp gas,” and in 1968, Blue Book concluded that a group of B-52 pilots who witnessed strange lights moving over North Dakota had simply seen the star Vega.

Among the many who reserved harsh words for Blue Book’s methods was none other than Dr. Hynek, who had been with the program since the days of Project Sign and was popularly viewed as its chief debunker. “The entire Blue Book operation was a foul-up,” he later wrote in the 1970s. “Not enough attention was paid to the subject to acquire the kind of data needed even to decide the nature of the UFO phenomenon.”

After Blue Book’s famous “swamp gas” explanation and other far-fetched attempts to move UFOs into the “identified” category, future President Gerald Ford—then a Michigan congressman—called for a “full-blown” Congressional investigation to “allay any apprehensions” that the Air Force was engaged in a cover-up. The result was an independent study on UFOs funded by the federal government and run out of the University of Colorado. Led by physicist Edward U. Condon, the group first convened in late 1966 before releasing its findings in a lengthy 1968 tome titled “Scientific Study of Unidentified Flying Objects.”

The Condon Report was unequivocal in its findings: “Our general conclusion is that nothing has come from the study of UFOs in the past 21 years that has added to scientific knowledge,” it read. “On the basis of present knowledge, the least likely explanation of UFOs is the hypothesis of extraterrestrial visitations.”

Critics claimed the study was biased—Condon himself described it as a “fiasco” and “damned nonsense”—but its findings convinced the Air Force to finally pull the plug on Project Blue Book. On December 17, 1969, the Secretary of the Air Force released a memo announcing that the study “no longer can be justified either on the ground of national security or in the interest of science.”

By then, Blue Book had analyzed 12,618 cases of flying objects in America’s skies, 701 of which remained “unidentified.” A few days later, a New York Times editorial said the decision to close Blue Book should be “applauded” as a victory for rationality. “No doubt the true believers will continue the quest,” the article added, “convinced more than ever that some bureaucratic conspiracy is seeking to hush up the news that the earth is under extraterrestrial reconnaissance.”

With the end of Project Blue Book, the federal government officially got out of the business of UFOs. President Jimmy Carter later suggested that NASA look into the subject in 1979, but the agency demurred on the grounds that there was not enough tangible evidence to warrant a study. Nevertheless, several other western nations have continued investigating. A “UFO desk” run by the British Ministry of Defense remained in operation until as recently as 2009, and France continues to keep an eye on the sky to this day under the aegis of GEIPAN, a government agency tasked with collecting and analyzing UFO reports.