The investigations were apparently conducted up until 1978, although the majority of the related files date back to the 1940s and 1950s.
The sightings of the alleged alien craft include unidentified objects spotted in eastern Germany, Spain, North Africa and over the uranium mines of the Belgian Congo.
In a rare display of humor, the members of the rather not-so-jolly US intelligence agency said that they have released "documents we think X-Files character Agent Fox Mulder would love to use to try and persuade others of the existence of extraterrestrial activity", as well as files that "his skeptical partner, Agent Dana Scully, could use to prove there is a scientific explanation for UFO sightings."
"The truth is out there; click on the links to find it," CIA officials wryly remarked.