Actually I think the "Truthers" (sic) are leftist. If 9-11 had happened on Clinton's watch, not one would be saying he was behind it. The only they're saying Bush, Rumsfeld, Cheney, et al, are involved in it is because they hate them because of their misperception of them as conservatives.
Freepers, who fancy themselves rightists, like to blame 9-11 on Clinton, saying he ignored Osama.
The belief in conspiracies depends on who you hate.
Believe me the truthers aren't largely comprised of 'leftists'; many are former left wing who have woken up to the phony left-right paradigm. I do think that as a whole, left wingers and democrats, as well as those not politically inclined, tend to be more receptive to the truth message. Old right and libertarians are already informed and don't need to be convinced of anything. the mainstream repubs and dumb neocon followers, who get their news from soundbytes, are the hardest to break through,.
In thinking the conspiratorial public positions the FBI has taken in the TWA flight 800 incident is merely illogical or incompetent. Our government wouldn't try and stop us from getting to the truth, would they?