Two giant 110 story buildings came crashing down after being hit by jets and sending flaming fuel and debris everywhere.
The impact would have wrecked the foundations from the shockwaves. Bldg 7 was on fire. The steel does not have to melt but soften to till it does not have the strength anymore as designed to hold up the load.
Nearly 3,000 dead and 11,000 wounded by the jets and the building debris.
PBS's Nova had a very good show explaining why the two main towers collapsed, and how it probably could have been prevented if they were built a couple of years later, when building codes were updated to require heat insulation on the steel hangers that supported each floor.
If I remember it correctly, the main towers had "hooks" which locked in to slots running vertically along the outer edges.
Due to the intense heat of jet fuel, the hooks were softened, lost their shape, and a whole floor collapsed on to the lower one.
Then because of the excess load from the weight of the collapsed floor, more hangars bent and floors collapsed until the building's framework failed.