It's my understanding that the fires from the jet fuel ignited the enormous stores of papers in the building.  

Plastics also burned.  The temperature inside the buildings rose enough to melt the slender steel support columns, and that led to the pancaking of the towers.  

The shockwave of a building about 1/4 mile high affected neighboring structures.  The pressure pulverized almost everything into a very fine powder that hung in the air for weeks.

Forensic teams studied the surviving columns and other structural materials quite extensively, and explosives experts also surveyed and analyzed the site.

In the trial of the bombers who tried to topple the WTC in 1993 by planting explosives in a basement parking garage, it came out that their intention was to knock that tower over on its side--then it could have killed tens of thousands of people.

After the 1993 attempt the authorities failed to conduct a thorough collection of data about the WTC and used that info in a probabilistic risk assessment which would have shown that the steel in the towers could not survive an intense fire.

Moi, I don't see what the difference is whether the WTC was destroyed by bombs or by planes. An attack is an attack.  And apparently it was far more successful than Osama had ever imagined.