He is considered by prosecutors to be the 9/11 mastermind, taking his hijacked planes idea to Bin Laden and later helping to recruit the pilots.
Born in Kuwait, he studied engineering in the US before fighting in Afghanistan in the 1980s.
Even before 9/11, the FBI were on his trail as they believed him to be behind other bombings and thwarted attacks.
In 2006, three years after being captured, he was sent to the US Navy base at Guantanamo Bay, Cuba, and has been held there since.
One reason he was not put on trial during that period was the fear that the brutal interrogation techniques used on him could have undermined the prosecution’s case.
Now he and two others held there have agreed to plead guilty to the murder of the 2,976 people listed in the charge sheet. In exchange they will not receive the death penalty.
The other two men are Walid Muhammad Salih Mubarak Bin Attash and Mustafa Ahmed Adam al-Hawsawi.
Some relatives of those killed on 9/11 are deeply unhappy about the plea deal, describing it as a victory for the conspirators.