Britain's defense ministry couldn't explain it either, but concluded that the unidentified flying object posed no threat.
The National Archives on Monday released the government's complete file on the "Rendlesham Forest Incident" of December 1980, one of Britain's most famous UFO sightings.
It was among more than 4,000 pages posted online Monday documenting 800 alleged encounters during the 1980s and 1990s. Over the past three years the Ministry of Defense has been gradually releasing previously secret UFO papers after facing Freedom of Information demands.
The Rendlesham file contains U.S. Air Force Lt. Col. Charles Halt's first-hand account of the event, which has been public knowledge for many years. The file includes the conclusions of a British government investigation and a letter from a former defense chief urging officials to take UFOs more seriously.
Halt reported that two servicemen had noticed "unusual lights" about 3 a.m. in the woods outside the gates of RAF Woodbridge, a U.S. base in eastern England. He wrote that patrolmen sent to investigate saw "a strange glowing object" in the forest.
The metallic, triangular object "illuminated the entire forest with a white light," he wrote.
The next day, investigators found depressions in the ground and unusual radiation readings. That night many personnel - including Halt himself - saw a pulsing "red sun-like light" in the trees that broke into five white objects and disappeared.
The Ministry of Defense could offer no definitive explanation for what the Air Force officers had reported seeing, but also found no evidence of "any threat to the defense of the United Kingdom."
Nothing had registered on radar, and "there was no evidence of anything having intruded into U.K. airspace and landed near RAF Woodbridge."
A 1983 letter in the file proposes a possible explanation involving a combination of the nearby Orford Ness lighthouse, a fireball and bright stars.
Case closed, as far as the ministry was concerned. But not everyone was convinced.
A 1985 letter from Lord Hill-Norton, former head of Britain's armed forces, to then-Defense Secretary Michael Heseltine, complained that the "puzzling and disquieting" episode had never been explained properly.
Hill-Norton said if the sighting was genuine, "British airspace and territory are vulnerable to unwarranted intrusion to a disturbing degree." The alternative explanation was that "a sizable number of USAF personnel at an important base in British territory are capable of serious misperception, the consequences of which might be grave in military terms."
Britain's defense ministry has charted UFO sightings since the 1950s, when a Flying Saucer Working Party was established. More files are due to be released by the archives through 2010.
Some of the newly released events came with easy explanations.
In 1993 and 1994, the ministry received numerous reports of a "brightly illuminated oval object" over London. It turned out to be an airship advertising a new car.
More mysterious was a UFO "attack" on a cemetery in Widnes, northwest England, in July 1996. A police report said a young man - "a sensible sort of lad and genuine" - reported seeing a UFO firing beams of light into the ground.
A police officer sent to the scene found a smoldering railway sleeper. "It does look rather odd," reported the officer, whose name was blacked out in the document.
The files include a little grist for conspiracy theorists.
The head of the ministry's UFO desk wrote briefing notes in 1993 reporting a spate of sightings in southwest England and speculating whether they might be connected to Aurora, a secret U.S. spy plane whose existence has never been officially admitted.
Atop one of his letters, someone scrawled: "Thank you. I suggest you now drop this subject."
The files reveal a 1996 spike in UFO sightings: 609 that year, up from 117 the year before.
David Clarke, a UFO historian and consultant to the National Archives, said it was probably no coincidence that the supernatural TV show "The X Files" was popular in Britain at the time, and that alien-invasion movie "Independence Day" came out the same year.
"It's evident there is some connection between newspaper stories, TV programs and films about alien visitors, and the numbers of UFO sightings," Clarke said.
"Aside from 1996, one of the busiest years for UFO sightings reported to the MoD (Ministry of Defense) over the past half century was 1978 - the year 'Close Encounters of the Third Kind' was released."