There will never be credible witnesses! What would make them credible, being a president, an astronaut, NORAD, military law enforcement, forget that.
Evidence for hackers, I think not, not all files are kept online.
Massive cover up like conspiracy theorists would say, not really, since even the players don't know the whole game or who the other players are. Very few if any would know the all the implications of all the different UFO types people witness, natural causes and otherwise. I'd venture to say that those who know enough also know it is best to leave it as mysterious things or hoaxes.