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: