* Legitimate terrorists who were allowed to go ahead and commit the acts.
* CIA oepratives taking part in terrorist drills but did not realise the thing was actually real. Once in the plane you can't really call the cops. Planes then remote controlled into targets or where ever.
This part of it all is un-certain. Doesn't matter though, only one link in a chain needs to break, that link is building 7 in this case along of course with many other things.
