Coordinates: .mw-parser-output .geo-default,.mw-parser-output .geo-dms,.mw-parser-output .geo-dec.mw-parser-output .geo-nondefault,.mw-parser-output .geo-multi-punct.mw-parser-output .longitude,.mw-parser-output .latitude40°42′42″N 74°00′48″W / 40.71167°N 74.01333°W / 40.71167; -74.01333

The collapse of the World Trade Center occurred during the terrorist attacks of September 11, 2001, after the Twin Towers were struck by two hijacked commercial airliners. One World Trade Center (WTC 1, or the North Tower) was hit at 8:46 a.m. Eastern time and collapsed at 10:28 a.m. Two World Trade Center (WTC 2, or the South Tower) was hit at 9:03 a.m. and collapsed at 9:59 a.m. The resulting debris severely damaged or destroyed more than a dozen other adjacent and nearby structures, ultimately leading to the collapse of 7 World Trade Center at 5:21 p.m. A total of 2,763 people were killed in the crashes, fires, and subsequent collapses, including 2,192 civilians, 343 firefighters, and 71 law enforcement officers as well as all the passengers and crew on the airplanes, which included 147 civilians and the 10 hijackers.

In September 2005, the National Institute of Standards and Technology (NIST) published the results of its investigation into the collapse. The investigators did not find anything substandard in the design of the twin towers, noting that the severity of the attacks was beyond anything experienced in buildings in the past. They 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 to buckle. Once the upper section of the building began to move downwards, a total progressive collapse was unavoidable.

The cleanup of the World Trade Center site involved round-the-clock operations and cost hundreds of millions of dollars. Some surrounding structures that were not hit by the airplanes still sustained significant damage, requiring them to be torn down. Demolition of the surrounding damaged buildings continued even as new construction proceeded on the Twin Towers' replacement, the new One World Trade Center, which was opened in November 2014.

 

Note:   The above text is excerpted from the Wikipedia article Collapse of the World Trade Center, which has been released under the GNU Free Documentation License.