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40°42′41.12″N 74°00′44.00″W / 40.7114222°N 74.0122222°W / 40.7114222; -74.0122222

The Twin Towers of the World Trade Center collapsed on September 11, 2001, as a result of being struck by two jet airliners hijacked by 10 terrorists affiliated with al-Qaeda, during the September 11 attacks. Two of the four hijacked airliners crashed into the Twin Towers, one into the North Tower (1 World Trade Center) and the other into the South Tower (2 World Trade Center). The collapse of the Twin Towers destroyed the rest of the complex, and debris from the collapsing towers severely damaged or destroyed more than a dozen other adjacent and nearby structures. The South Tower collapsed at 9:59 am, less than an hour after being hit by the second hijacked airliner, and at 10:28 am the North Tower collapsed. Later that day, 7 World Trade Center collapsed at 5:21 pm from fires that had started when the North Tower collapsed. As a result of the attacks to the towers, a total of 2,763 people died. Of the people who died in the towers, 2,192 were civilians, 343 were firefighters, and 71 law enforcement officers. Aboard the two airplanes, 147 civilians and 10 hijackers also died.

Immediately following the attacks, a building performance study (BPS) team of engineering specialists was formed by the Structural Engineering Institute of the American Society of Civil Engineers (SEI/ASCE) and Federal Emergency Management Agency (FEMA). The BPS team issued its report in May 2002, finding that the aircraft impacts caused "extensive structural damage, including localized collapse" and that the resulting fires "further weakened the steel-framed structures, eventually leading to total collapse". They also presented recommendations for more detailed engineering studies of the disaster.

The BPS team investigation was later followed by a more detailed investigation conducted by the National Institute of Standards and Technology (NIST), which also consulted outside engineering entities. This investigation was completed in September 2005. The NIST investigators did not find anything substandard in the design of the WTC towers, noting that the severity of the attacks and the magnitude of the destruction was beyond anything experienced in U.S. cities in the past. They also emphasized the role of the fires and found that sagging floors pulled inward on the perimeter columns: "This led to the inward bowing of the perimeter columns and failure of the south face of WTC 1 and the east face of WTC 2, initiating the collapse of each of the towers."

The cleanup of the site involved round-the-clock operations, many contractors and subcontractors, and cost hundreds of millions of dollars. The demolition of the surrounding damaged buildings continued even as new construction proceeded on the World Trade Center's replacement, One World Trade Center, which was structurally completed on May 10, 2013, when the final component of the spire was installed atop the skyscraper. As of 2014, One World Trade Center, 4 World Trade Center and 7 World Trade Center have been replaced.

1. "Bin Laden claims responsibility for 9/11". CBC News. October 29, 2004. Retrieved January 11, 2009. al-Qaeda leader Osama bin Laden appeared in a new message aired on an Arabic TV station Friday night, for the first time claiming direct responsibility for the 2001 attacks against the United States.
2. "Security Council Condemns, 'In Strongest Terms', Terrorist Attacks on the United States". United Nations. September 12, 2001. Retrieved September 11, 2006. The Security Council today, following what it called yesterday's "horrifying terrorist attacks" in New York, Washington, D.C., and Pennsylvania, unequivocally condemned those acts, and expressed its deepest sympathy and condolences to the victims and their families and to the people and Government of the United States.
3. "PartIIC – WTC 7 Collapse" (PDF). NIST Response to the World Trade Center Disaster. National Institute of Standards and Technology. April 5, 2005. Retrieved November 1, 2006.
4. Foderaro, Lisa W. (September 11, 2009). "9/11's Litany of Loss, Joined by Another Name". The New York Times. Retrieved August 29, 2010.
5. Corley, G.; Hamburger, R.; McAllister, T. (2002), McAllister, T. (ed.), "Executive Summary" (PDF), World Trade Center Building Performance Study: Data Collection, Preliminary Observations, and Recommendations, ASCE/FEMA
6. "NIST Response to the World Trade Center Disaster" (PDF). Retrieved April 23, 2008.