Who did it?

There were 19 hijackers who took control of the four planes. The man that linked them was called Mohamed Atta. He was in the first plane to crash into the World Trade Center.

In February 2003 a man called Mounir al-Motassadek was sent to prison for 12 years for helping some of the hijackers before the attacks, when they were living in Germany.

In May 2006, a man called Zacarias Moussaoui was found guilty in a US court of being involved in the 9/11 attacks. He was sent to prison for the rest of his life.

A man called Osama Bin Laden is thought to have been behind the attacks.

Militant group

Shortly after 11 September 2001, a video was released reportedly showing Bin Laden laughing and boasting about the attacks.

He claimed to have known that they were going to happen and spoke of his joy at seeing the towers collapse.

The US government has been searching for him ever since, but so far has been unable to capture him.

Bin Laden is the leader of al-Qaeda, a militant Islamic group thought to have been behind a number of other attacks on US targets, including embassies in African countries Kenya and Tanzania.