9/11

A terrorist attack on 11 September 2001 that killed a large number of people became known as 9/11. (PROSE: I Was a Monster!!!, The Danger Men)

The Laughing Gnome sent William Bishop's soul from 2011 into Calum Bence's body in 1999. Bishop had to overcome the urge to warn the authorities about 9/11, which was two years in the future. There were budget cuts to secret service espionage in 1999, but Bishop knew that changed after 9/11. (PROSE: The Danger Men)

In 2002, the Capricorn Killer wrote in his diary: "I want - no, I need - to be 9/11 not Big Brother."

In a 2003 book of essays, a historian wrote: "Following the attacks of September 11th 2001, we all know the problems of the world aren’t so easily defined, let alone solved." (PROSE: The Gallifrey Chronicles)

Mark Whitaker was a man from 2011 sent back in time to 1994 by the Weeping Angels. In 2003, he said to the Eleventh Doctor: (PROSE: Touched by an Angel)

"All the good people who have died, where I've stood by and done nothing. How do you think I felt on September the 11th, watching all those people die? But I did nothing."

- Mark Whitaker

In 2005, (PROSE: Rise of the Dominator) Emily Chaudhry and Francis Currie remembered the 11 September attacks; the media wondered if the attacks were due to British involvement in Iraq. (AUDIO: The Longest Night)

Behind the scenes
The 9/11 terrorist attack was carried out by al-Qaeda and destroyed the World Trade Center, although these facts are not explicitly mentioned in DWU sources.