The collapse of the World Trade Center was the central event of the September 11, 2001 attacks. The two main towers of the World Trade Center (WTC) complex were each hit by aircraft. The south tower (2 WTC) collapsed at 9:59 a.m., less than an hour after being hit, and the north tower (1 WTC) followed at 10:28 a.m.
2,751 people inside and near the towers were killed, including all 157 passengers and crew aboard the two airplanes. The collapse of the twin towers also caused extensive damage to the rest of the complex and nearby buildings. As a result, at 5:20 p.m. 7 World Trade Center collapsed as well.
The Federal Emergency Management Agency (FEMA) completed its performance study of the buildings in May 2002. It declared the WTC design sound and attributed the collapses wholly to extraordinary factors beyond the control of the builders. While calling for further study, FEMA suggested that the collapses were probably initiated by weakening of the floor joists. When the floors detached from the main structure of the building and fell onto each other, said FEMA, a progressive "pancake" collapse began.
FEMA's proposed explanation was rejected by a subsequent and much more detailed investigation by the National Institute of Standards and Technology (NIST), which was completed in September 2005. Like FEMA, NIST vindicated the design of the WTC, noting that the severity of the attacks and the magnitude of the destruction was beyond anything experienced in U.S. cities in the past. But NIST did not attribute the collapses to failing floor joists, but rather found that sagging floors pulled the outer walls of the buildings inwards until they buckled.
The cleanup of the site involved round-the-clock operations, many contractors and sub-contractors, 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, the Freedom Tower. Of the destroyed buildings, only 7 World Trade Center has been replaced as of 2008.
