Langley AFB, Virginia. I was stationed at Eglin AFB in Ft Walton Beach, FL, but was in Va with my boss and my boss's boss (both civilians, boss was retired USAF, his boss was a military brat married to an AF retiree). I'd been in the USAF 15 years at that point and was a long way from Salem, Indiana. I pictured a Cessna hitting the building when it came over the radio, found out different a few minutes later. We had to switch rental cars at Norfolk IAP. The Navy fleet was out in force in Chesapeake Bay, the airport was a ghost town.
After the 18 hour drive home the next day (nothing flying 12 Sept) I drove in the east gate at Eglin (from Niceville, we lived on base) about 2300 after dropping the other two off: light carts were putting spotlights out like crazy. There was a Peacekeeper (armored Dodge Ramcharger) with an M-60 mounted on top, I could see the ammo belt hanging from it. I remember thinking that the world we knew would never be here again.