9/11 terror attack anniversary: Here's what happened on September 11, 2001 in the United States:
Nineteen al-Qaeda terrorists hijacked four US passenger airplanes.
Between at 8 and 9 am, two of the planes crashed into the twin towers of the World Trade Center complex in Manhattan. Within two hours, both the 110-storey towers collapsed.
The debris of the twin towers led to a collapse of all other buildings in the World Trade Center complex, including the 47-storey 7 World Trade Center tower.
The third plane was crashed into the headquarters of the US Department of Defense - Pentagon in Virginia.
The fourth plane was crashed into a field in Pennsylvania after passengers tried to fight back against the hijackers.
The United States responded by launching a "War On Terror" in Afghanistan to depose the Taliban, which had failed to comply with US demands to extradite the founder of al-Qaeda, Osama bin Laden.
The destruction of the World Trade Center affected the economy of Manhattan and had a significant effect on global markets.
While the cleanup of the World Trade Center site was completed in May 2002, the Pentagon building was repaired within a year. The construction of One World Trade Center began at the World Trade Center site in 2006 and it opened to the public in 2014.
Several memorials have been built in memory of the victims of Septemebr 11 attacks. These include the National September 11 Memorial and Museum in New York City, the Flight 93 National Memorial in a field in Stonycreek Township in Pennsylvani and the Pentagon Memorial in Arlington County in Virginia.
The 9/11 attack anniversary brings back haunting memories of the fateful day. People pay tribute to the victims of the attack on this day and also pledge to fight against terrorism.