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.
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?