Please listen to this Canadian interview of the Danish scientist Niels Harrit on 26 Feb 2011
At about 48-50 minutes into this 56 min talk, you can see a relatively closeup video of a WTC tower collapse. Looking under the "mushroom canopy" you can plainly see squibs igniting in rapid fire proceeding down the building. Also not the outward trajectory of beams (even upwards). Gravity cannot cause that. A fire cannot cause that. A plane impact could cause that but not in a delayed manner (102 minutes wrt WTC#1[North Tower] or 40 minutes WTC#2[South Tower]). Explosives or incendiary material can.
WTC#7 was quite obviously a traditional controlled demolition (video fairly early in this interview). That has to be pre-wired with charges put at strategic location and a timer set for each charge -usually proceeds up the building for traditional demolitions. That takes typically a couple of weeks after a building had been evacuated. So since this building was pre-wired w/o anyone knowing it (building was still in use and not evacuated until that morning), the Twin Towers could also have been pre-wired.