How much is true?

Much of the movie is based upon real events dramatized for effect. Washington was real. Mantell was killed chasing a UFO. The color films are genuine. The actors played real characters, though some real UFO witnesses were on camera.

Except the character of the reporter. But all the incidents are well documented, and the film at the end pretty much speaks for itself.

There was a reason for the flying saucer craze back then - they were seeing something in the skies.