In June 1947, while flying his small plane, businessman and civilian pilot Kenneth Arnold reported seeing nine objects moving at high speeds through the skies over Washington’s Mount Rainier. Widely publicized reports of Arnold’s experience, followed by an increasing number of reported UFO sightings, led the U.S. Air Force to begin an investigation into the sightings, called Operation Sign, in 1948.

The initial investigation resulted in the formation of Project Blue Book in 1952; that project became the longest running of the U.S. government’s official inquiries into UFO sightings, compiling reports on more than 12,000 sightings or related events from 1952 to its dismantling in 1969.

READ MORE: Interactive Map: UFO Sightings Taken Seriously by the U.S. Government

Early Sightings

Though reports of mysterious flying objects–often attributed to spirits, angels, phantoms, ghosts or other supernatural phenomena–have existed for centuries, World War II and the accompanying development of rocket science marked a new level of interest in what would officially become known as unidentified flying objects (UFOs). The first well-known UFO sighting occurred in June 1947, when civilian pilot and businessman Kenneth Arnold reported seeing nine objects, glowing bright blue-white, flying in a “V” formation at speeds of up to 1,700 mph in the skies over Washington’s Mount Rainier.

Did you know? Kenneth Arnold compared the movement of the nine mysterious objects over Mount Rainier to that of "a saucer if you skip it across water." This statement later led to the misconception that the objects were shaped like saucers, and to the widespread use of "flying saucer" as a synonym for UFO.

After news of Arnold’s experience hit the media, a rash of similar sightings were reported across the United States, including a highly controversial report of what appeared to be a crashed UFO near a U.S. Army base in Roswell, New Mexico. (The Army claimed the object in question was the wreckage of a weather balloon, claims that conspiracy-minded “ufologists” would later dispute.) In response to the increasing number of UFO-related reports, the U.S. Air Force launched Operation Sign in 1948. Among the initial theories of the project’s participants was that some UFOs were actually Soviet aircraft (this was the Cold War era, after all), although they also posed the hypothesis that some might be extraterrestrial spacecraft.

READ MORE: When a U.S. Fighter Pilot Got into a Dogfight with a UFO

Formation of Project Blue Book & the Robertson Panel

The Air Force’s UFO-related inquiries took place against a backdrop of frenzied popular interest in the strange flying objects, which reached its peak soon after Project Blue Book began in 1951. Headquartered at Wright-Patterson Air Force Base in Ohio, Project Blue Book would become the longest running of the U.S. government’s official inquiries into UFOs. Alarmed by the striking number of UFO sightings reported in 1952, the administration of President Harry S. Truman feared an outbreak of hysteria over the issue. In 1953, the Central Intelligence Agency (CIA) responded to these fears by assembling an expert panel of scientists, headed by physicist H.P. Robertson of the California Institute of Technology, to discuss the UFO issue.

READ MORE: When UFOs Buzzed the White House and the Air Force Blamed the Weather

The Robertson Panel met for three days, during which they interviewed military officers and Blue Book officials and reviewed photos and film of supposed UFOs. The panel concluded that there was no basis for the so-called extraterrestrial hypothesis, and that UFOs posed no security threat. Fully 90 percent of the sightings, according to the Robertson Panel, could be attributed to astrological or meteorological activity, or to man-made causes such as balloons or searchlights. The panel’s findings were not fully declassified until 1979, feeding suspicions that a government conspiracy was in the works.

READ MORE: Meet J. Allen Hynek, the Astronomer Who First Classified UFO 'Close Encounters'

The Condon Report

Over the next 17 years, Project Blue Book would compile reports of 12,618 UFO sightings or related events. Similarly to the Robertson Panel, Blue Book would eventually classify more than 90 percent of these as “identified,” meaning they were caused by a known astronomical, atmospheric or artificial (man-made) phenomenon. The remaining 700 incidents remained “unidentified”; these included cases in which there was insufficient information to assign the event a known cause.

In 1966, the Air Force had requested the formation of another committee to look into the details of 59 UFO sightings investigated by Project Blue Book. The committee, headed by Dr. Edward Condon and based at the University of Colorado, released its “Scientific Study of Unidentified Flying Objects”–better known as the Condon Report–in 1968. According to the Condon Report, the sightings they examined showed no evidence of any unusual activity, and recommended that the Air Force stop investigations into UFO-related incidents. In 1969, in response to the Condon Report as well as a declining number of UFO sightings, Project Blue Book was officially brought to an end; among its conclusions were that of the sightings categorized as “unidentified,” there was no evidence submitted to or discovered by the Air Force that they were the result of technology beyond the range of modern scientific knowledge or that they were extraterrestrial vehicles.

“Ufology” Continues

Despite the dismissive attitude expressed by the Condon Report and the subsequent dismantling of Project Blue Book, civilian investigations into UFOs continued, as many “ufologists” were dissatisfied with the government’s conclusions. In 1974, the astronomer J. Allen Hynek, who had served as an adviser to Project Blue Book, created the Center for UFO Studies (CUFOS). This organization continues to look into UFO sightings and to weigh the hypothesis that they could be evidence of extraterrestrial activity.

In addition to UFO investigations conducted in the United States, similar work has been done over the years in other countries all over the world, including Canada, the United Kingdom, Australia, Greece and Sweden. In January 1979, the British House of Lords even held a three-hour-long debate on the subject of UFOs and a motion (eventually defeated) that the British government should make public what it knew about them.

Don't miss the return of Project Blue Book, Tuesday January 21 at 10/9c on HISTORY.