The idea that interceptor aircraft on scene could have totally prevented this attack is a fantasy. Imagine a fighter tracking the first plane heading for New York City. Now what? You don't KNOW that he's going to hit the WTC, or any other building, although it's a possibility. No hijacker has ever done that before. You do know that the plane is full of passengers. Hell, you can see them looking at you out the windows. Are YOU going to blow it out of the sky? Based on what?

As far as I know, there were no standing orders to down such hijacked aircraft, and who the hell would give such an order? And had they shot the planes down before any buildings were hit, making the only casualties of September 11 the plane's occupants, the pilot's hide would still be hanging on some wall in Washington, never mind the hide of the President or whoever gave him the order.

There might be some credibility to the idea that later airliners might have been shot down after the first plane or planes hit their targets, thus firmly demonstrating the reality of the situation. Many conspiracy-minded people still think that this is what happened to the flight over Pennsylvania, although there is no real evidence of this.