There are 9/11 conspiracy nuts who are nuts and then there's German politician Andreas von Bülow, a member of the center-left Social Democrats who spent 25 years in the Bundestag and was Helmut Schmidt's Minister for Research and Technology for a few years in the '80s.
von Bulow: CIA blew up the World Trade Center
Von Bülow's The CIA and September 11 (Die CIA und der 11.September) blames the CIA for directly causing the 9/11 deaths and destruction. Der Spiegel's Christian Stöcker has a good piece on the guy -- the excellent German news magazine has notably debunked him time and again, especially one of his claims that the 9/11 hijackers are in fact still alive. But like those "zombie terrorists," as Der Spiegel derisively called them, von Bülow's ideas won't stay dead, and his book still sells. He remains reasonable-sounding and may in fact be the only mainstream-type guy who is greatly admired by both left-wing and right-wing radicals.
His theory is at least interesting, considering that he rightly points out that the U.S. government has pathologically lied about so much stuff, so why not this? In his view, the CIA didn't screw up and allow 9/11 to happen. The spy agency actually did the bad deeds. As if the CIA could get it together enough to pull something like that off.