In the aftermath of the collapse, the questions came quickly and urgently: was it inevitable that such a severely damaged structure would completely collapse? Or was there something about the building's design or construction that made it especially susceptible to failure? What exactly caused that failure-fire, heat, internal damage? Was the evacuation plan adequate enough to save the maximum number of lives? And are other tall buildings just as vulnerable to collapse when the blast is great and the fire uncontrolled? In the gray wasteland of smoke and dust and mangled steel that was once the third tallest building in the world, forensic engineers began sifting the wreckage for clues and answers to these and other structural truths. This blue-ribbon team of engineers is nearing the end of its task, and NOVA has been there from the beginning, following its investigation of the causes of the calamity