On 11 September 2001, al-Qaeda operatives hijacked four passenger aircraft, flying two into the Twin Towers of New York's World Trade Center, which collapsed, and one into the Pentagon near Washington; the fourth crashed in Pennsylvania after passengers resisted. Nearly 3,000 people were killed, the deadliest terrorist attack in history. The attacks triggered the US-led 'war on terror' and far-reaching changes in security policy and surveillance. They also forced a long debate over how to rebuild the destroyed site in Lower Manhattan.
The attacks ushered in a security-and-surveillance turn in urban design — bollards, vehicle setbacks, hardened lobbies, pervasive CCTV and 'defensible' public space — and drove the protracted, contested rebuilding of the World Trade Center site.
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