The Roswell incident, in July 1947, went down in UFO folklore after the US Air Force announced it had found the remains of what was suspected of being a crashed flying saucer in the New Mexico desert near the town.
The next day it retracted the statement amid claims it was later identified as a weather balloon.
UFO chasers have forever since believed it was a major cover-up and that the US military actually discovered the craft and the bodies of alien pilots.
The FBI memo now in the spotlight, dated from 1950 - three years after the Roswell incident - said each of three "saucers" contained three bodies of three-foot tall humanoid occupants, making nine aliens in total.
Museum workers stumbled across the document as as they were preparing to open the Hakui Centre for UFO Research in the coastal city of Hakui, 310 miles west of Tokyo, Japan, back in 1994.
But it has only come to light again after subsequent documents released under the US Freedom of Information Act suggested the FBI was so concerned about the museum opening, it opened up its own file on the proposals.
The original 1950 memo suggested the US military found three metallic, saucer-shaped objects, complete with occupants, near New Mexico, in the USA.
The memo revealed US radar equipment in the area was believed to "have interfered with the saucers and brought them down", according to the Journal Telegraph.
It said: "An investigator for the Air Forces states that three so-called flying saucers had been recovered in New Mexico.
"They were described as being circular in shape with raised centres, approximately 50 feet in diameter.
"Each one was occupied by three bodies of human shape but only three-feet tall, dressed in metallic cloth of a very fine texture."
The memo was sent by Federal special agent Guy Hottel, who was in charge of the FBI Washington Field Office, on March 22, 1950, to then FBI Director J Edgar Hoover. Continue reading