09:59 South tower collapses
Once one floor had melted the collapse became inevitable, as each new falling floor added to the pressure on the floor below. Further down the building, even steel at normal temperatures gave way under the enormous weight - an estimated 100,000 tonnes.
A little less than an hour after the second crash, the south tower, which had been the second tower to be hit, crumbled to the ground. Hundreds of firefighters, police officers and people who were trying to escape the towers were crushed.