A former Pentagon official who led a secret government programme to combat UFO attacks says he believes aliens have visited Earth.
Luis Elizondo previously ran the Advanced Aerospace Threat Identification Programme which, leaked records show, investigated reports of clashes with unidentified aircraft.
The former chief of the US$22 million ($31.4m) project says he thinks "we may not be alone", based on numerous reports he read on encounters with the flying craft, according to the Daily Mail.
Many reportedly displayed out-of-this world speed and manoeuvring capabilities.
News of the off-the-books Department of Defence (DoD) programme emerged over the weekend.
It sought to identify what had been seen, either through tools or eyewitness reports, and then "ascertain and determine if that information is a potential threat to national security".
Elizondo resigned from the DoD in October in protest over what he has termed excessive secrecy and internal opposition to the project. Its funding was cut in 2012.
He has now said there is evidence that leads him to believe alien aircraft have visited the planet.
Speaking to CNN, he said: "My personal belief is that there is very compelling evidence that we may not be alone.
"These aircraft - we'll call them aircraft - are displaying characteristics that are not within the US inventory nor in any foreign inventory that we are aware of.
"Things that don't have any obvious flight services, any obvious forms of propulsion, and maneuvering in ways that include extreme manoeuvrability beyond, I would submit, the healthy G-forces of a human or anything biological."
Among the sightings were reports from pilots of two US Navy Super Hornet fighters who spotted a UFO on a training mission in 2004.
They were 160km from San Diego in the Pacific when a call on their radios asked if they were carrying weapons.
The unusual request came from the naval cruiser, Princeton, which had spent two weeks tracking unidentified aircraft.
Commanders David Fravor and Jim Slaight had only dummy missiles, but were directed to investigate objects that appeared suddenly at an altitude of 24,000m, then plunged towards the sea.
At 6000m, they stopped and hovered before disappearing out of radar range or shooting up again.
The pilots could see nothing at first and then Fravor looked down to the sea.
The water in one place was being churned by something just below the surface.
Hovering erratically 15m above it was some sort of flying craft, about 12m long, oval-shaped and whitish.
As the pilot descended towards it, it rose to meet him, but suddenly peeled away at an immense speed that he admits left him feeling "pretty weirded out".
The craft "had no plumes, wings or rotors" but, seemingly travelling at about a kilometre a second, easily outran America's fastest military jets.
Fravor's comrades made fun of him when he described the encounter, but others in the US military, we now know, took him seriously.
For the episode was one of scores of unexplained encounters between military personnel and UFOs that were investigated by a top-secret, multimillion-dollar programme run by the Pentagon.
Although it was set up in 2007, the existence of the Advanced Aviation Threat Identification Programme (AATIP) has only now emerged, thanks to its former boss.
Its US$22 million of funding — so-called "black money" for secretive projects — was known only to a few outsiders.
UFO enthusiasts have argued for decades that the US Government has been covering up the existence of unidentified craft containing alien visitors.
The idea that a hush-hush Government outfit was investigating sightings and other bizarre phenomena famously provided the basis for TV drama series The X-Files.
Now, it seems the cult series wasn't such a flight of fancy after all.
The shadowy programme's existence was intentionally buried in the defence department's $600 billion annual budget, as were its headquarters, deep within the labyrinthine Pentagon building.
Based on the fifth floor of C Ring, the secret department has spent years investigating reports of unidentified flying objects.
Although the Pentagon officially stopped funding the project in 2012, insiders told the New York Times it was still operating.
And, more tantalisingly, intelligence experts who ran it, and politicians who backed it, insist its research has not been fruitless.
Having investigated myriad reports from US servicemen of encounters between unknown objects and military planes, they are convinced that nothing in this world can explain them.
"If anyone says they have the answers now, they're fooling themselves," said Harry Reid, the US Senate Democrat leader for 12 years and the project's most powerful supporter. "We do not know."
Reid, who retired recently as senator for Nevada, first directed the Pentagon to investigate the "unidentified aerial phenomena" repeatedly identified.
In each case, the servicemen were convinced that what they saw was vastly more technologically advanced than anything in US or foreign arsenals.
The man who inspired this "X-Files department", Reid, had the support of two other senior senators, both members of a defence spending sub-committee, who feared a threat to national security behind these chilling sightings. Their rationale was that if the mysterious craft were not aliens, then perhaps Russia or China had developed advanced technology to threaten the West.
Reid's interest in UFOs had originally been pricked by his friend Robert Bigelow, a billionaire hotel tycoon and government contractor who is investing millions in space projects such as inflatable modules for living on the moon.
Bigelow, who became convinced extra-terrestrials exist after his grandparents said they saw a UFO, has been investigating the paranormal for decades and bought a Utah ranch known for UFO sightings in the skies above.
The Pentagon UFO programme paid Bigelow's Las Vegas-based aerospace research company to do most of its work.
Reid says he was also influenced by the veteran astronaut John Glenn, who had told him years earlier that the Government should be seriously looking into UFOs and talking to military people who claimed to have seen them. Too often, their claims were not being passed up the chain of command because servicemen feared they would be ostracised. The Pentagon programme investigated scores of reported encounters — in some cases, such as Fravor's, backed by video or audio evidence.
Newly released tapes make for disturbing listening. In another incident involving a US Navy Super Hornet jet chasing a UFO that emitted a "glowing aura travelling at high speed and rotating as it moves", a pilot is heard exclaiming: "There's a whole fleet of them . . . My gosh, they're all going against the wind. The wind is 120 knots to the west."
Suspiciously, sightings were often near nuclear facilities, be they ships or power plants. In many cases they involved aircraft that appeared to defy the laws of physics in their speeds and manoeuvrability. Often, they were able to move or hover with no visible means of propulsion or lift.
Seeking explanations, the Pentagon focused attention on other phenomena that sound as if they've come from a sci-fi convention. They included warp drives (faster than light spacecraft propulsion), and wormholes (theoretical passages in space-time that could create shortcuts).
Researchers also analysed people claiming to have experienced physical effects from encounters.
The Pentagon investigators are likely to have talked to some of the 120 retired military personnel who — according to UFO researchers — have described encounters near nuclear missile bases. Some believe aliens were monitoring them to ensure humanity didn't blow itself up by accident.
They include Air Force captain Robert Salas, an intercontinental ballistic missile launch officer on duty at Malmstrom Air Force Base in Montana one night in 1967.
He says he was warned by his men "screaming into the phone" that a mysterious "glowing red object" had been spotted over their missile silo, which was 18m underground. Moments later, they discovered that all 10 Minuteman missiles had been deactivated.
Robert Jamison, the base's targeting officer, confirmed the report and said he heard about a UFO landing in a "deep ravine" nearby. He said he spoke to a security guard, who described "two small red lights off at a distance" that began to close in. The guard then broke down and started crying.
More recently, the department has assessed the threat posed by UFOs and Bigelow has modified some of his company's buildings to store materials reportedly recovered from the scene of UFO sightings.
Those involved insist they made progress. In 2009, Reid wrote to then deputy defence secretary William Lynn requesting heightened security to protect the programme. "Much progress had been made with the identification of several highly sensitive, unconventional aerospace-related findings," he said.
At the same time, a Pentagon briefing by Elizondo claimed that "what was considered science fiction is now science fact", says the New York Times.
He warned that the US was incapable of defending itself against the technologies that had been discovered, although he conceded none of the UFOs showed "overt hostility". A project insider told the website Politico, however, that the programme couldn't justify using taxpayers' money. It lost its funding.
Elizondo quit in October, in protest at what he said was excessive secrecy and internal opposition to his work. But the Pentagon insisted it would act "whenever credible information is developed".
The US has investigated UFOs before, notably in 1949 when it launched a 20-year study — Project Blue Book — into more than 12,000 sightings. Although 701 were never explained, the report attributed most to people seeing conventional aircraft or spy planes, stars and clouds.
Many will laugh at the US Government wasting so much time and money on them. But others think they were on to something.
The Navy pilot who says he saw that astonishing craft off San Diego, Fravor, told ABC News yesterday: "I can tell you, I think it was not from this world. I'm not crazy, haven't been drinking. After 18 years of flying, I've seen pretty much about everything I can see in that realm, and this was nothing close.
"I have never seen anything in my life, in my history of flying that has the performance, the acceleration — keep in mind this thing had no wings".
And Reid is sticking to his guns. "I'm not embarrassed or ashamed or sorry I got this thing going. I've done something that no one has done before," he said.
As news of his UFO secret emerged, he even borrowed The X-Files' famous catchline, tweeting: "The truth is out there."