"2 planes, 3 buildings.

the 'pancaking' of the WTC towers would take 96 seconds for the energy in them to disperse by its own weight.

dropping a coin off the top would take about 9 seconds to hit the ground.

it only took about 10 seconds for the buildings to fall. they behaved as though they had the tensile strength of air.

this is because their tensile strength was compensated for with explosives. there is no other way to concentrate the energy required to destroy those buildings in the time it took except for outside sources of power."
--wbur forums

Thermate was found on the WTC steel. This would be sufficient to provide that energy. Someone placed demolition explosives on most every floor of the WTC before the planes struck. The WTC were undergoing 24-hour security in the weeks before 9/11. On one day, bomb sniffing dogs were suspiciously removed.

Either these several hundreds of installations of explosives were performed before the security began, months before 9/11, by terrorists, who then waited for the planes to hit and detonated them, or they were put in on the day the bomb sniffing dogs were removed, or a slim chance exists that they were put in by terrorists in a deceptive way under the cover of security and dogs, and that security did not find the hundreds or thousands of explosives during their weeks of patrols, which is a very low liklihood. On any day that bomb sniffing dogs were there, they would have smelled the explosives entering the building unless they were well sealed. The force necessary to detonate the buildings would require tons of thermate, and hundreds of manhours to install, and building-intimate time sensitive activity on every floor.

It is extremely unlikely that terrorists performed this activity, and it is certain that explosives took the towers down.