The thing is all these conspiracy theories fall apart when you actually look at them logically. When a tragedy like this happens, the natural human reaction is to assume some complicated plot behind it.
People make up these conspiracy theories (with zero evidence) as the simple explanation is actually quite remarkable. No one wants to believe that a lone gunman could have killed the US president, so all these ridiculous conspiracy theories come up like "The Mafia had JFK shot because he was sleeping with Marilyn Monroe", "The Cubans had Kennedy shot and set Lee Harvey Oswald up to make it look like he'd done it". When the simple truth is that Oswald shot Kennedy because he was a nutter.
The same applies here. You can say all you want that the Americans carried out 9/11 by themselves to give a reason for invading Iraq, but there is no conspiracy. If there was, they'd have made it look like it was the Iraqis that did it. The simple truth is that a group of nutters blinded by the ideology of a madman hijacked a few planes and flew them into buildings.