From the Norad tapes: 9:24 a.m. The FAA notifies Neads of a third aircraft south west of D.C.

At about 9:24 a.m. (channel 7) Scoggings called Neads from the Boston Center and mentioned an unknown aircraft south west of Washington D.C.

direct link

"we did hear from Washington that there is an aircraft - they believe is American 11 - and he is south west of Washington"

"they're missing a third... we don't know who is the third one now ..there's a third aircraft missing.."

9:24 is the notification time listed in the Norad timeline release on September 18th, 2001...

..and mentioned by Colonel Scott to the 9-11 Commission on the march 23rd, 2003 hearings