they keep them on alert 24/7, and they did before 911.
there were exactly 100 flights intercepted in the 365 calendar days before 9/11/01.
Aside from the idiot's reason that "they were all training that day", there is the problem of the fighters which WERE dispatched. They were zooming in on planes at 1875 mph and would have cut the fckers down very quickly. However, *something* told them to cut speed to 400 mph and do a holding pattern in the atlantic.
We'd know which force was able to stop them from their mission, except the FAA supervisor destroyed all his tapes. Then - hilariously? - NORAD said they didn't have any capability or responsibility to intercept planes inside the US - even though they were actually running drills about intercepting domestically hijacked flights in the US that very morning!