Battalion Chief Tim Lasher of Reagan National notices a crack in the structure and Captain Dennis Gilroy sees a cornice of the Pentagon move. Captain Michael Defina warns of imminent collapse and urges everyone near the impact site to evacuate.1

1 Goldberg, A. Papadopoulous, S., et al “Pentagon 9/11” Historical Office, Office of the Secretary of Defense n.d PDF. March 17, 2011 page 70 http://osdhistory.defense.gov/docs/Pentagon%209-11.pdf