Just to clarify...
As Gravy pointed out NEADS were not notified that UA93 had been hijacked until after it had already crashed. Furthermore, no shoot-down order was received by NEADS until after the attacks had finished.
Following sensible protocol, NEADS determined that pilots would not be issued general shoot-down authority. Instead, each individual incident would be addressed after a successful interception had been made.
None of the four aircraft were successfully intercepted on 9/11, nor would it have been realistically possible for any of them to have been successfully intercepted on 9/11. Thus the issue of a shoot-down order is entirely moot.
Data recovered from UA93's black box indicates that the passengers never got past the cockpit door, that the hijackers intentionally crashed the aircraft into the ground, and that it was functioning normally upon impact.
At 1003, when UA93 crashed, the fighter defense status is as follows:
1) A pair of F-15C Eagles from Otis ANGB, scrambled by NEADS in response to the hijacking of AA11, is flying a CAP over Manhattan
2) A pair of F-16 Falcons from Langley AFB, scrambled by NEADS in response to an incorrect report of AA11 heading to Washington DC, are flying CAP over Washington DC
3) Unarmed fighters from Selfridge Field, Michigan are being scrambled to intercept Delta 1989, a suspected hijacking.
4) Fighters from the 121st FS, based at Andrews AFB, have been scrambled by the Secret Service to maintain a CAP over Washington DC in anticipation of UA93's arrival. Although unarmed, the aircraft have been given permission to use force, and pilots are considering ramming any hijacked airliners.
None of the sets of aircraft mentioned above was anywhere near Shankesville at 1003.
-Gumboot
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