The September 11 attacks (also referred to as September 11, September 11th or 9/11) were a series of four coordinated suicide attacks upon the United States in New York City and the Washington, D.C., area on September 11, 2001. On that Tuesday morning, 19 terrorists from the Islamist militant group al-Qaeda hijacked four passenger jets. The hijackers intentionally crashed two planes, American Airlines Flight 11 and United Airlines Flight 175, into the Twin Towers of the World Trade Center in New York City; both towers collapsed within two hours. Hijackers crashed American Airlines Flight 77 into the Pentagon in Arlington, Virginia. The fourth jet, United Airlines Flight 93, crashed into a field near Shanksville, Pennsylvania, after passengers attempted to take control before it could reach the hijacker's intended target in Washington, D.C. Nearly 3,000 died in the attacks.
Suspicion quickly fell on al-Qaeda, and in 2004, the group's leader Osama bin Laden, who had initially denied involvement, claimed responsibility for the attacks. Al-Qaeda and bin Laden cited U.S. support of Israel, the presence of U.S. troops in Saudi Arabia, and sanctions against Iraq as motives for the attacks. The United States responded to the attacks by launching the War on Terror, invading Afghanistan to depose the Taliban, which had harbored al-Qaeda members. Many countries strengthened their anti-terrorism legislation and expanded law enforcement powers. In May 2011, after years at large, bin Laden was found and killed.
The destruction caused serious damage to the economy of Lower Manhattan and had a significant impact on global markets. Cleanup of the World Trade Center site was completed in May 2002, and the Pentagon was repaired within a year. Numerous memorials were constructed, including the National September 11 Memorial & Museum in New York, the Pentagon Memorial, and the Flight 93 National Memorial. Adjacent to the National Memorial, the 1,776 feet (541 m) One World Trade Center is estimated for completion in 2013.
Source: Wikipedia, "September 11 attacks", available under the CC-BY-SA License.