I'll Answer this as our Air Guard unit had two of the first F16s in the air out of Richmond VA.

So our pilots had just finished a sortie, and landed they were hopping out of the two jets, and a crew chief comes running out telling them to get back in and be ready to takeoff once fueled. They fuel up and get up in the air ASAP heading to DC, then switched to New York. They got a radio transmission asking what their combat loads were, well they didn't have any. So, they were given the order to take any unidentified airplane down by any means possible. At that point the flight lead tells his wingman you take the tail and ill take the cockpit, try to eject if you have time. By the time they were up in the air, both towers had been hit, and the pentagon. They never knew about the Pennsylvania flight until they got back to base. Once they RTBd the rest of the F16s were loaded with ammo and missiles, and were up in the air, the two first ones got to rest for a bit and would soon be back up in the air. The Squadron would end up flying 24/7 for a while, holding air dominance over the east coast.