By Brian Todd, CNN
Forget about gangsters and bank robbers, the most popular document in old FBI case files has to do with UFOs.
It's called "The Vault," the FBI's digital reading room where any of us can go online and view the bureau's most notorious cases.
Guess which is the most popular file.
John Dillinger's? Jimmy Hoffa's?
"Since we opened the vault, it's been this memo about flying disks or flying saucers, and it relates to an allegation that we heard from a third hand, saying that the Air Force had found a couple of saucers out in the New Mexico desert," said FBI historian John Fox.
No, no can't be. I mean most people want to read about Machine Gun Kelly and Al Capone, right?
"You would think so, but this memo itself has gotten over a million page views in two years since we put it up. Al Capone doesn't make our top 50," said Fox.
The memo's all of two paragraphs. Agent Guy Hottel, then head of the FBI's Washington field office, writes that an Air Force investigator "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."
Not only that...
"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 flyers and test pilots."
[Reporter]: "This was never followed up on right?
"No, in fact it says right here, 'No further evaluation was attempted concerning the above," said Fox.
[Reporter]: "Why not?"
"From what's written here, from what we can read, it certainly looks like they thought this was third-hand information, that this was not necessarily a hoax, which it could well have been, but that someone was simply reporting hearsay," said Fox.
And it was more for the air force to look into, along with countless other reports of UFOs in Roswell, New Mexico and elsewhere. Reports that were never substantiated.
One reason the memo from Agent Hottel went viral is because when the FBI vault was set up online two years ago, tabloids seized on that memo, saying it appeared to back up theories that aliens exist.
And it's not just the Guy Hottel memo that's a favorite. There are hundreds of other pages of memos and files in the FBI vault in the 'unexplained phenomenon' section, that are more popular online than the FBI's files on Bonnie and Clyde, serial killer Ted Bundy and other famous cases.
Fox says out of all the strange cases he's come across...
"The descriptions here of you 50-foot diameter saucers and human-shaped three-foot tall metallic-clothed aliens, that's unique," said Fox.
And, we can say, a little frustrating for FBI officials, who say it diverts attention from all the work they've done, all the dangers they've faced through the years to capture fugitives and solve the nation's most difficult crimes.