US military prosecutors have filed charges against the alleged mastermind of the 11 September 2001 attacks on the US.
If convicted, the US will seek to have Khalid Sheikh Mohammed and five other detainees executed.
The charges against the men were announced in an news conference at the Pentagon this afternoon.
They will be the first charges from the Guantanamo war court alleging direct involvement in the attacks and the first involving the death penalty.
Prosecutors will send the charges to a Pentagon appointee overseeing the Guantanamo trials, Susan Crawford, whose approval is needed before any trials can proceed.
Mr Mohammed, a Pakistani national, confessed in custody that he planned every aspect of the 11 September attacks.
However his confession could be problematic if used as evidence because the CIA has admitted it subjected him to a simulated drowning technique known as 'waterboarding' during interrogations.
The procedure is widely considered to be torture and the Guantanamo court rules prohibit the use of evidence obtained through torture, as does an international treaty the US has signed.
The charges against Khalid Sheikh Mohammed will include conspiring with al-Qaeda to attack and murder civilians and about 3,000 counts of murder for those killed on 11 September.
Mr Mohammed was arrested in Pakistan in March 2003 and handed over to the United States. He is one of 15 'high-value' al-Qaeda prisoners previously held in CIA custody and later sent to Guantanamo, most of them in 2006.