Newswise — More than 2,500 people lost their lives in the collapse of the Twin Towers, but the disaster could have been much worse.

It’s likely that the terrorists’ goal was to knock the skyscrapers over, which would have widened the disaster and perhaps killed thousands more, says William Bulleit, a professor of structural engineering in the Department of Civil and Environmental Engineering at Michigan Technological University.

Instead, the towers were initially able to remain standing after the planes’ impact. Eventually, however, the impact of the crashes and the ensuing fires weakened their steel framework. This caused the floors above the crash level to pancake downward, leading to the towers’ collapse.

The ability of each tower to withstand the initial impact and continue to stand, for almost an hour in one case and over an hour in the other, allowed a large number of people to escape the towers prior to collapse, saving many lives.