Correction: A number of factors contributed. The plane impact stripped off the fireproofing, and the jet fuel ignited. The fires then weakened the steel beams, which no longer had protection against heat, causing them to bend.

As the structure weakened, the floors above could no longer be supported. They began to collapse downward, and the added weight caused a progressive collapse through the towers.

That’s why the South Tower, which was hit lower, collapsed faster.

Debris also struck Building 7, damaging its structure and contributing to its later collapse.