And Professor Stephen Jones and a colleague proved it was nano-thermite. Nano-thermite is a tiny-sized form of thermite that can do a tremendously great amount of destructive force for a small amount of thermite.
Youtube search on "Stephen Jones thermite."

The buildings were weakened with directed energy, there was some explosive work but none that could make it go free fall speed. Essentially the metal was altered at the molecular level, forgive me if that sounds dumb I'm not a physicist.