Barack Obama: Generations will never forget September 11
15 May 2014 Last updated at 16:32 BST
President Barack Obama has said a September 11 memorial museum would ensure that that future generations will "never forget" those who lost their lives in the attack.
The National September 11 Memorial Museum includes thousands of personal items and parts of the World Trade Center towers themselves.
Almost 3,000 people died on 11 September 2001 after al-Qaeda hijackers flew aeroplanes into the towers.