I'm not saying it DIDN'T happen. I'm saying that I find it difficult to believe. Conspiracy theorists are going from weird crop circles to "OMG, Space Aliens from Europa and Ganymede are bombing Omaha, Nebraska!"
Again, it's not to say it couldn't or didn't happen. Just that I can't see a clear motive and then see such coordination in attack (plant explosives and/or thermite, hijack four civilian planes and crash them, then get hundreds of participating people to keep quiet about it all.)