Puzzles of Twin Tower and 7th Building

The collapse of twin tower had been considered unusal right after the incident. Many stories are over the medias regarding possible mistries behind it.
What is considered unsually is the speed of the collapse. Both towers fell down with the same speed as that of an free fall object. Structures of lower floors did not provide any resistence to the fall.
Insurance fraud has been one of the allegations behind the incident. According to " Is There A Secret To The 9/11 Insurance Battle? ", A major insurance company is holding back from paying Larry Silverstein insurance money...
Explosions were heard before the collapse of twin tower. Video evidence shows that there were explosions at the base of tower 1.
There had been as many historical events as mystries. The form is for people to talk about during tea time, the later for historians to dig around and create theories.