http://www.whitehouse.gov/news/releases/2001/09/20010916-2.html

George W. Bush Press Conference 16 September 2001

[…]

Q            Mr. President, would you confirm what the Vice President said this morning, that at one point during this crisis you gave an order to shoot down any civilian airliner that approached the Capitol? Was that a difficult decision to make?

             THE PRESIDENT:  I gave our military the orders necessary to protect Americans, do whatever it would take to protect Americans.  And of course that’s difficult.  Never did anybody’s thought process about how to protect America did we ever think that the evil-doers would fly not one, but four commercial aircraft into precious U.S. targets – never.  And so, obviously, when I was told what was taking place, when I was informed that an unidentified aircraft was headed to the heart of the capital, I was concerned. [Who informed the President ? When ? If this was known, why was nothing done to stop the aircraft ?]  I wasn’t concerned about my decision; I was more concerned about the lives of innocent Americans.  I had realized there on the ground in Florida we were under attack.  But never did I dream we would have been under attack this way.

             That’s why I say to the American people we’ve never seen this kind of evil before.  But the evil-doers have never seen the American people in action before, either – and they’re about to find out.