An odd remnant of the core of the north tower remained standing
for about 15 seconds after the main collapse, then seemed to abruptly
disintegrate into a narrow column of dust where
huge steel box columns had been...
In this view the spire can be seen in the wake of the debris cloud just
as it reaches the ground. (And incidentally at that moment the scene to
the left of the dust cloud gets much brighter, a brightness visible in
many images. Its possible source is beyond the scope of this discussion.)
This remnant of the core remains standing for a little less than 20 seconds,
swaying a bit from side to side but showing remarkably little inclination to
topple over. It then abruptly begins to drop straight down on itself in
an apparent free fall, but after falling for about a quarter of its height
it suddenly turns to dust. It is heavy dust that continues to drop straight
down in place with very little dispersion or wind drift, suggesting that the
particles were dense, more or less what one would expect to see if the steel
of the columns had turned all at once into a coarse powder.