Within hours of the attacks, suspicion quickly fell onto al-Qaeda. The United States formally responded by launching the war on terror and invading Afghanistan to depose the Taliban, which had not complied with U.S. demands to expel al-Qaeda from Afghanistan and extradite its leader, Osama bin Laden. The U.S.'s invocation of Article 5 of NATO—its only usage to date—called upon allies to fight al-Qaeda. As U.S. and NATO ground forces swept through Afghanistan, bin Laden fled to the White Mountains, where he narrowly avoided capture by U.S.-led forces. Although bin Laden initially denied any involvement, in 2004 he formally claimed responsibility for the attacks. Al-Qaeda's cited motivations included U.S. support of Israel, the presence of U.S. troops in Saudi Arabia, and sanctions against Iraq. After evading capture for almost a decade, bin Laden was killed by the U.S. military on May 2, 2011.
The attacks resulted in 2,977 non-hijacker fatalities, an indeterminate number of injuries, and substantial long-term health consequences, in addition to at least $10 billion in infrastructure and property damage.It remains the deadliest terrorist attack in human history and the single deadliest incident for firefighters and law enforcement officers in U.S. history, with 343 and 72 killed, respectively. The destruction of the World Trade Center and its environs seriously harmed the New York City economy and induced global market shocks. Many other countries strengthened anti-terrorism legislation and expanded their powers of law enforcement and intelligence agencies. Cleanup of the World Trade Center site (colloquially "Ground Zero") took eight months and was completed in May 2002, while the Pentagon was repaired within a year. After delays in the design of a replacement complex, the One World Trade Center began construction in November 2006 and opened in November 2014.Memorials to the attacks include the National September 11 Memorial & Museum in New York City, the Pentagon Memorial in Arlington County, Virginia, and the Flight 93 National Memorial at the Pennsylvania crash site.
