There were credible and eyewitness reports that they had terrorists IN THE "JUMP" seats of other planes, but due to the oh-so-familiar departure delays, they were still on the ground when the first 4 planes went down and airspace was shut down. These 'pilots' scrambled off the planes and out of sight.
Reports name the Capitol as one target, Hoover Damn and others.
So they were successful, but not totally
One time to be deeply grateful for airport delays. :-/