The man named Khalid Sheikh Mohammed has been described as one of al-Qaeda leader Osama bin Laden's closest men.
He was arrested on suspicion of the September 11 attacks in Pakistan in 2003 and has been held at the US military base in Guantanamo Bay since 2006. The drawn-out legal process has been affected by the question of whether torture he and others were subjected to in CIA custody has affected the evidence.
In the 2001 attacks, terrorists hijacked commercial airliners and crashed them into the World Trade Center in New York and the Pentagon. Nearly 3,000 people died in the attacks.
