The memo from March 22, 1950, written by the head of the FBI's Washington office, Guy Hottel, has been viewed nearly a million times in the past two years, according to the FBI's website.
It is the most popular file in the FBI Vault, a website which posts Bureau documents released under the Freedom of Information Act.
The Hottel memo has actually been a part of the public record since the 1970s, but it was not widely read until it was added to the FBI Vault in 2011, NBC News reported.
The memo said an informant told an FBI agent that an Air Force investigator had given him details about recovered UFOs.
"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 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." - FBI memo
Hottel said the saucers' control systems could have hit interference from "a very high-powered radar set-up" installed by the government nearby.
The informant's name has been blacked out on the document.
According to the FBI, the Hottel memo, written three years after a rumored 1947 UFO sighting in Roswell, New Mexico "does not prove the existence of UFOs; it is simply a second- or third-hand claim... 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."