Flash and Destruction of NE corner, WTC2 around Fl 90
https://www.youtube.com/v/the6Ns6IpuE?version=3&feature=player_detailpage
Three good gif animations of the event
The flash and break along the northeast corner allows the upper portion
of the building to fold into the lower section of the building as shown:
https://www.youtube.com/v/YQkmqDxp6YE?version=3&feature=player_detailpage