While the "truther" movement has been traditionally been a leftist conspiracy theory movement, I'm starting to think that it is now becoming more and more conservative-friendly. First it was the Ron Paulites then a candidate for the Republican gubernatorial primary, now it's another shooter nut, aka a teabagger terrorist.
Conservatives are in their CYA mode over the Bush presidency and many of them view him as a leftist and not a real conservative. So it's plausible that the same people who believe that Obama is a secret commie Muslim born in Kenya, believe that Bush staged 9/11. Conservatives are motivated by anti-government beliefs and the truther movement is about as anti-government as you can get.
These nuts don't fool me, they are all conservatives.