The Vault section of the FBI's website contains many declassified documents related to reported UFO sightings as well as formerly top secret memos about unusual objects discovered near Roswell, New Mexico. The site contains more than 2,000 digitized, searchable files.
The files go back as far as the 1940s, when the Air Force began investigating reports of flying discs. As an example, the files contain information about the reported discovery of 3-foot-tall beings dressed in metallic clothing. Another document contains a description of a high-altitude weather balloon with a radar deflector, previously thought to have been a UFO, discovered in Roswell.