I'm a conspiracy kook:  I think a bunch of guys in a cave planned this, then they hijacked 4 planes with plastic box cutters (overwhelming former military fighter pilots), flew hundreds of miles undetected for 2 hours and no jets scrambled, and then, without any help from radar or ATC, found and struck their targets with pinpoint precision.  Then three buildings fell straight down.  Yeah, that's the most likely scenario.  Wake up.