A UFO that buzzed warships off the coast of California for almost a week was so advanced, it couldn't have belonged to any nation on Earth, a leaked Pentagon report claims.
The Tic Tac-shaped object was seen on six different days by crewmen aboard the USS Princeton and USS Nimitz in 2004, and footage of one of the encounters was made public late last year.
Since then speculation has been rife as to what the strange object was. The Pentagon has refused to release any documents from its UFO research unit, which closed in 2012, but a team of investigators from US CBS-affiliated news station KLAS managed to get their hands on some.
According to the documents, in 2009 an in-depth analysis of the 14m-long UFO found it was able to turn on a dime at extreme speeds and make itself invisible to radar.
"It takes off like nothing I've ever seen," retired Navy pilot David Fravor says. "One minute it's here, and off, it's gone."
It also appeared to be communicating with a "large disturbance under the surface of the ocean" between 50 and 100m across.
Seven F-18 pilots and a number of radar operators on the Nimitz and other US Navy vessels confirmed the presence of the UFO, which in the report is called an AAV, or Anomalous Aerial Vehicle.
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In addition to eyewitness and radar accounts, a naval sensor capable of tracking a golf ball at 160km recorded its flight path and speed.
The report said it was "no known aircraft or air vehicle currently in the inventory of the United States or any foreign nation".
The report was buried after the witnesses and its authors faced ridicule, KLAS reports, and the UFO deemed to not be any threat.
A second report written a few years later was also reportedly buried, as was a third.
Luiz Elizondo, who ran the Pentagon's Advanced Aerospace Threat Identification Programme (AATIP), told KLAS anyone who believes the witnesses mistook birds and weather balloons for advanced craft are mistaken.
"Let the data speak for itself," he told KLAS. "Let the information we receive from electro optical data, electromechanical mechanisms be the tool in which we look and compare what the eyewitness testimony is saying."
Previously released AATIP documents show the Pentagon had been investigating craft with no visible forms of propulsion or lift.