Why did they do it?

The way America has got involved in conflicts in regions like the Middle East has made some people very angry, including a group called al-Qaeda - who are widely thought to have been behind the attacks.

In the past, al-Qaeda leaders have declared a holy war - called a jihad - against the US. As part of this jihad, al-Qaeda members believe attacking US targets is something they should do.

When the attacks happened in 2001, there were a number of US troops in a country called Saudi Arabia, and the leader of al-Qaeda, Osama Bin Laden, said he wanted them to leave.