For most people the image of the collapse of the World Trade Center Towers on September 11, 2001, was not only a scene of unforgettable horror, it was a moment of unimaginable consequence. Within days, NOVA began following a blue ribbon team of forensic engineers as they began searching for clues that would tell them why the towers fell.

This moving and informative documentary features interviews with survivors and rescue personnel who recount the buildings' last moments and their harrowing journeys to safety, interweaving these stories with the insights of some of the leading structural engineers in the world to explain exactly what happened on that fateful day. Why the Towers Fell takes viewers through the process by which the investigative team came to understand the how's and why's of one of America's greatest tragedies. From a detailed examination of the building's original design to the relentless process of combing scrap steel yards and Ground Zero itself for evidence, this was one of the most extensive and difficult disaster investigations ever undertaken. The team tested building materials, calculated the role of the jet fuel in the fire, estimated the speed of the aircraft and the damage to the building's core, and they analyzed the effectiveness of the escape and fire protection systems. The conclusions they reached will certainly influence the building of future skyscrapers for years to come.