On September 11, 2001 a series of four terror attacks took place against the US, including two hijacked planes that crashed into the World Trade Center.
Islamic terrorist group al-Qaeda claimed responsibility for the attacks.
Two decades on, the city of New York has reopened the Cortlandt Street subway station, which was buried under debris when the World Trade Centre collapsed.
Elsewhere in the US city, there are reflective reminders of the attacks, such as Ground Zero, which pays tribute to those who lost their lives.
Here, we take a look back at the numbers behind the 9/11 tragedy:
Victims killed in the attacks: 2,977
The age range of the victims:2-85
Number of British victims: 67
Percentage of victims who were men:70-80
The number of minutes respectively each tower stood burning: 56, 102
Nations who lost citizens killed in the attacks:115
The number of seconds the towers took to fall:12
The maximum heat of fires, in degrees Fahrenheit, at World Trade Center:2,300
The number of survivors rescued from ground zero:0
The estimated speed of flight 175 in mph as it crashed into the South tower: 590
Days underground fires at the World Trade Centre continued to burn:69
Death certificates issued without a body at the request of victims' families: 1,616
Days the New York stock exchange was closed: 6
World Trade Center victims' remains that have been positively identified (out of 2,753): 1, 642
Hours taken to clean up 1.8 million tons of debris:3.1 million
Estimated amount of money it cost to plan and execute the attacks:$500,000
Estimated cost of World Trade Center site damage: $60 billion
The percentage by which PTSD increased in New Yorkers after 9/11: 200