Out of all the case files made public by the FBI online, the most popular is a memo from 1950 titled "FLYING SAUCERS," the agency said this week.
The mysterious report from Guy Hottel, special agent in charge in Washington, begins with this:
"An investigator for the Air Force stated that three so-called flying saucers had been recovered in New Mexico. They were described as being circular in shape with raised centers, approximately 50 feet in diameter. Each one was occupied by three bodies of human shape but only 3 feet tall."
The name of the source is blacked out.
So did the bureau take it seriously?
"It certainly looks like, they thought this was third-hand information," said FBI historian John Fox. Either it was the result of a hoax, he said, or "someone was simply reporting hearsay."
Still, over a million people have looked at the sensational memo online.
The account goes on to say that the bodies were "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 memo also relays a potential explanation for why the flying saucers were found in New Mexico.
"The Government has a very high-powered radar set up in that area," the allegation says, "and it is believed the radar interferes with the controlling mechanism of the saucers."
The FBI took no further action on the case.
Although the file was first released in the 1970s, it was posted online in 2011 as part of the Vault, unleashing a flood of tabloid headlines about little green men. In an article headlined " 'Aliens exist' say real-life X-Files," the London tabloid The Sun said the memo appeared to back the claim that extraterrestrials landed in Roswell, New Mexico.
But in a new commentary posted this week, the FBI said that since this memo was dated three years after the supposed Roswell landing, "there is no reason to believe the two are connected."
The memo is part of a cache of hundreds of pages of accounts under the heading "Unexplained Phenomenon," describing claims of UFO sightings, spacecraft debris and alien landings.
Also posted in the Vault are hundreds of files on some of the FBI's most storied criminal cases such as Al Capone and Bonnie and Clyde and John Dillinger.
It also includes entertainers: Marilyn Monroe and Frank Sinatra and Tupac Shakur.
Many of the files chronicle cases in which agents used painstaking methodologies and high-risk raids to solve crimes and capture dangerous fugitives.
By comparison, Fox says, "the descriptions here of you know, 50-foot diameter saucers and human-shaped three-foot tall metallic-clothed aliens -- that's unique."