Concept
The collapse of the World Trade Center occurred on Tuesday, September 11, 2001, after two commercial airliners hijacked by Al-Qaeda terrorists were deliberately flown into the Twin Towers of the World Trade Center complex in New York City as part of the September 11 attacks. The North Tower (WTC 1) was the first building to be hit when American Airlines Flight 11 crashed into it at 8:46 a.m., causing it to collapse at 10:28 after burning for one hour and 41 minutes. At 9:03 a.m., the South Tower (WTC 2) was struck by United Airlines Flight 175; it collapsed at 9:58 a.m. after burning for 55 minutes. The towers' destruction caused major devastation throughout Lower Manhattan, and more than a dozen adjacent and nearby structures were damaged or destroyed by debris from the plane impacts or the collapses. Four of the five remaining World Trade Center structures were immediately crushed or damaged beyond repair as the towers fell, while 7 World Trade Center remained standing for another six hours until fires ignited by raining debris from the North Tower brought it down at 5:21 that afternoon. The hijackings, crashes, fires and subsequent collapses killed an initial total of 2,760 people. Toxic powder from the demolished high-rises was dispersed throughout the city and gave rise to numerous long-term health effects that continue to plague many who were in the towers' vicinity, with at least three additional deaths reported. The 110-story towers are the tallest freestanding structures ever to be destroyed, and the death toll from the attack on the North Tower represents the deadliest terrorist act in world history. In 2005, the National Institute of Standards and Technology (NIST) published the results of its investigation into the collapse. It found nothing substandard in the towers' design, noting that the severity of the attacks was beyond anything experienced by buildings in the past. The NIST determined the fires to be the main cause of the collapses, finding that sagging floors pulled inward on the perimeter columns, causing them to bow and then buckle.