On September 11th, 2001 members of an Al-Qaeda cell in the United States of America highjacked four commercial airliners. The hijackers took flight classes, learning just to fly planes and not to land them.
The Twin Towers, at the New York City World Trade Center, has several architectural flaws which increased the death toll from the attacks. The most prevalent defect was that the three staircases were constructed towards the center of the building, rather than being at the corners of the building. Therefore, hundreds of employees at the north and south towers at the floors north of the points of impact were trapped and unable to escape.
A 9-11 Memorial & Museum was established after the attacks to honor those who died, those who were wounded, the emergency responders, as well as the thousands of individuals mourning a lost family member from the attacks.