The previously classified records, which contain information of extraterrestrial encounters dating back 60 years, show eye-witness accounts of flying saucers and alien life forms.
In one document, a special agent in charge of the FBI's Washington Field Office sent a letter to the FBI's director on 22 March 1950, claiming that an Air Force investigator recovered a spacecraft in a field near to Roswell, New Mexico.
It reads: "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."
The report, which is registered on the FBI's online Vault library, then goes on to describe the 3ft human-like bodies of the aliens among the remains of the UFO landing.
According to the document, each foreign body was dressed in a fine metallic cloth and bandaged in a manner similar to the blackout suits used by speed flyers and test pilots.
The information comes after the Ministry of Defence released thousands of pages of UFO material at the National Archives last month.
Speaking, Nick Pope, a former UFO investigator for the British Ministry of Defence commented on the reports, saying: "These documents are extremely interesting, but it's going to be very difficult to discover the truth after so many years. There are many questions for the FBI and the United States Air Force, but the problem is that nobody now working for these organisations will have been employed at the time of the incidents described."
"Even if we don't solve the mystery of UFOs and the Roswell crash, these papers show that the American government took a strong interest in the paranormal. Maybe 'The X-Files' is closer to the truth than people think."
David Hardy, an official from the FBI's Records Management Division said the declassification of secret files reflects a strong commitment to build public trust and confidence through greater public access to FBI records.
He said: "The new website significantly increases the number of available FBI files, enhances the speed at which the files can be accessed, and contains a robust search capability."