Creative Speculation

You realize the materials hoisted into the air and installed in the towers, every last bit of it, stores potential kinetic energy to be released when it can fall, right? En total, the amount of energy released by the collapse of the towers, IGNORING the fires, was on the order of one percent of a small atomic bomb.

Less than 1% of that energy was converted to seismic energy, and each tower still produced a 2.0 on the Richter scale. About 30% of that energy went into breaking apart the materials of the building. The remaining ~70% is converted into heat. Heat located in a concrete oven-like environment that retains heat excellently well, in the rubble pile.

Every floor was 3.2 million pounds of concrete and steel. A layer of concrete 4 inches thick. You break that up by smashing it and dropping at least some of it 100+ floors, you generate a lot of heat. 10^12 Joules is a shedload of energy. Energy is never created nor destroyed, only converted.