A declassified document from 1949 tells how three men, on patrols miles apart, each reported seeing a UFO break up over mountains north of Salt Lake City.
The 1950 memo that recounts the discovery of flying saucers and aliens in New Mexico
Their extraordinary accounts were sent to Edgar Hoover, the director of the FBI, in a memo titled "Flying Discs".
The file said that a policeman, a highway patrolman and an army guard all "saw a silver coloured object high up approaching the mountains at Sardine Canyon" that "appeared to explode in a rash of fire."
"Several residents [reported] seeing what appeared to be two aerial explosions, followed by falling object," it said.
The file is one of thousands of previously unreleased files that the FBI has made public in a new online resource called The Vault.
Among them is a 1950 statement from special agent Guy Hottel, which seems to provide evidence for the theory that aliens landed at Roswell, New Mexico.
In the memo, Agent Hottel said that "three so-called flying saucers had been recovered in New Mexico".
He wrote that the flying saucers were "described as being circular in shape with raised centers, approximately 50 feet in diameter."
In a statement that is reminiscent of the hit 1990s TV programme The X-Files, he went on to describe the alien life forms inside the UFOs.
"Each one was occupied by three bodies of human shape but only 3 feet tall," he wrote.
The bodies were "dressed in a metallic cloth of a very fine texture. Each body was bandaged in a manner similar to the blackout suits used by speed flyers and test pilots."
Agent Hottel suggested that the UFOs had crashed in New Mexico because high-powered government radar had interfered with their operating systems.
The release of the files are likely to intensify the claims of conspiracy theorists who believe the US government covered up alien landings.
Roswell became infamous after reports that a flying saucer had crashed in the desert nearby in 1947.
The bodies of aliens were said to have been recovered by the military.