LONDON, England (CNN) -- Thousands of documents about reported UFO sightings -- ranging from calm accounts by professional pilots to unhinged rants about the extraterrestrial menace -- have been released by the British Ministry of Defence.

Taiwan resident Lee Chun-hung took these pictures showing a ball of fire trailing across the sky.

The 4,500 pages cover sightings that were reported from 1986 through 1992. The British military released them to a curious public as part of a four-year project to transfer all such documents to the National Archives.

One highlight from the batch released Monday involves the captain of an Italian airliner. He shouted "Look out!" to his co-pilot in April 1991 after claiming to see a beige "missile-shaped object" shoot past the cockpit.

In that instance, the defence ministry ruled out a missile and "all the usual explanations," wrote David Clarke, a UFO expert and journalism instructor at Sheffield Hallam University, who worked with the National Archives to prepare the new materials for release.

"The end result was this was a genuine UFO and the file was simply closed," he wrote. "There was nothing more they could do."

The newly released documents also carry an account by a U.S. Air Force pilot who says he was told to shoot down an unidentified flying craft over eastern England. But before he could fire, the object disappeared.

Don't Miss Secret UFO files released by UK government

The next day, a man arrived to debrief the pilot and "he was told in no uncertain terms that what he had seen on his radar was top secret and he wasn't to speak about it to anyone," Clarke wrote.

The first set of files was made available to members of the public in May. It covered reported UFO sightings from 1978 to 1987, and included hundreds of police reports taken from witnesses who described seeing lights or strange objects in the sky.

People who reported having seen UFOs typically describe various shapes and colors of lights, moving in formation or hovering in the sky. Witnesses reported orange, red, white and green lights that were diamond-shaped, square, or cigar-shaped.

They reported them to police, who have a standard 16-question form specifically for UFO sightings.

"The vast majority of them are just ordinary people who've seen something unusual and thought that they ought to tell someone about it," Clarke has said.

The Ministry of Defence said it examined the reports solely to determine whether enemy aircraft had infiltrated British airspace. Once it was determined that no enemy aircraft were in the sky, it did not investigate further.

"The Ministry of Defence has no other interest or role regarding UFO matters and does not consider questions regarding the existence or otherwise of extraterrestrial life-forms," it said in May.

That left many incidents unexplained.