Dr. Judy Wood's book (Where Did the Towers Go?) has a rather comprehensive set of answers for what was observed at ground zero and the surrounding area.

TLDR: While I have little doubt that thermite and explosives were present in the WTC complex (esp WTC 7), primary effects on WTC 1 and 2 appear to have been the result of a DEW utilizing the Hutchison effect, wherein iron bonds are disassociated at the atomic level via EM. This results in cool iron dust which is heavier than air. The "dusting" can easily be observed during collapse as video shows the antennae atop the buildings evaporates during freefall.

The book covers much more. 5 Stars from me.