A 1950 memo of an unconfirmed UFO sighting is the most popular file in the FBI's Vaults, having been viewed almost one million times.
The one page memo is stored electronically in the Bureau's records and was released under the Freedom of Information Act.
The FBI said that since its release two years ago, the file has become extremely popular with its online users.
Written by Guy Hottel, the then-head of the FBI's field office in Washington DC, the memo is dated 22 March 1950 and is addressed to the FBI director, J Edgar Hoover.
Hottel served in this position until the following year, when he took a job in the Identification Division. He retired in 1955 when he was in his 50s.
The memo recounts a story told to an agent by a third party, who said an Air Force investigator had recovered three flying saucers in New Mexico.
"They [the saucers] 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.
"Each body was bandaged in a manner similar to the blackout suits used by speed fliers and test pilots."
The third party said the saucers were found because the government's radar had interfered with the controlling mechanisms of the saucer.
However, the memo ends by saying "no further evaluation was attempted".
When the FBI Vault was launched in 2011, some media reports suggested the memo confirmed proof of a UFO crash at Roswell.
However, the Bureau says this is not the case as the memo is dated three years after the events at Roswell were said to take place.
The FBI also notes that it investigated UFO accounts in the years that followed Roswell (1947) but stopped this practice in July 1950.
"The Hottel memo does not prove the existence of UFOs; it is simply a second- or third-hand claim that we never investigated. Some people believe the memo repeats a hoax that was circulating at that time, but the Bureau's files have no information to verify that theory," the FBI said.
The Vault file on Roswell is also available online and can be viewed here.