In this video from Bill Cooper's early days, Bill discusses his discoveries while in Naval Intelligence, and his book "Behold A Pale Horse".
It should be noted that while Bill states in this interview that he believed "alien beings" to be real, and visiting from another planet, he later came to the conclusion that this was most likely NOT the case, and that the UFO information he read while in the military was meant to deceive the public into believing an alien threat existed.
This is part 1 of a 4 part interview. (less info)