http://thewebfairy.com/911/closeup.wmv
(detail, one frame per second)
As the collaspe reaches the white spot highlighted by the arrow, suddenly the collapse explodes horizontally.
Notice that the final frame here shows another white spot destined to cause another bout of horizontal explosiveness.
See also:
http://thewebfairy.com/911/shockwave
to see the tremendous shockwave the helicopter filming this footage endured
after catching the historic footage.
My source file, from Christian
Czech in Germany, is
http://thewebfairy.com/video/collected/TTVideo.avi
Here is a clip of the same scene
with a fig-leaf crawl over the white spot seen in advance of the collapse. This
video is also very dark and has poor contrast, tho the German version obtained
recently, from which these frames are made, has fine color.
http://thewebfairy.com/video/collected/tower_collapses.mpg