Follow on attack more than likely...and folks before we get cocky and smug.... and think we are out of the woods going on 6 years out from 9/11....Al Qaeda likes to revisit targets they didn't get the first time.
He's lying, I think. If memory serves, they determined that the plane that hit the Pentagon was might initially have been trying to hit the WH -- they made a couple of circuits of DC before heading off to the Pentagon.