Competent spies, criminals, and computer hackers don't leave a lot of evidence. I'd say that's a pretty good measure of competence at those activities. Flatly refusing to entertain the possibility that something like this might happen without solid proof is just as bad as mindlessly believing every fantastical nutter hypothesis without question. It may even be worse because those people don't sound crazy, but historically they've been wrong as often as the obvious loons.