At the heart of the towers were vertical steel and concrete cores, housing lift shafts and stairwells. The towers were built to be tough enough to withstand the impact of a plane. The steel was covered in concrete to guarantee firefighters a minimum period of one or two hours in which they could operate if a fire developed.

But the fires on 11 September were fed by more than 91,000 litres of jet fuel. The steel cores within the towers heated up to above 800C and the protective concrete cladding on the cores could only keep the heat at bay for a short time. Once the steel frame on one floor had melted, it collapsed, inflicting massive forces on the already-weakened floor below.