An American flag. I’m surprised this angle didn’t get more attention… although the edit takes away from the shot. It may be a network camera. Also found as an mpeg1 with other news from 9/11… at archive.org in the impact collection, specifically in this video file.
NIST Cumulus file: NBC Atlanta4 70 (69 is pre-collapse, also repeat air in files 80-81)
NBC 9/11 2:59 PM American Flag (partial)
YouTube 1
