The Pentagon has cleared the way for a death penalty trial against five Guantanamo Bay inmates charged with engineering the 11 September 2001 terrorist attacks on the United States.
Retired navy vice admiral Bruce MacDonald, who is in charge of military commissions, has signed off on the capital trial against alleged mastermind Khalid Sheikh Mohammed, 46, and four accused co-conspirators.
The other four men facing the death penalty charges are from Yemen, Saudi Arabia and Pakistan.
They all face charges in the system set up by former president George Bush within months of the attack, and then modified by President Barack Obama in 2009.
It will be up to an 11 member team of US prosecutors to prove the charges to a military jury of a dozen or more members.
If convicted, the five could be sentenced to death.
A total of 2976 people died when hijacked airliners struck in New York, Washington DC and Shanksville, Pennsylvania on 9/11.
President Barack Obama came into office in 2009 pledging to close Guantanamo Bay and try high-value terror suspects in US civilian courts.
But the administration reversed its intentions in April 2011 after widespread opposition.
The five were eventually charged in June 2011 with offences similar to those they were accused of by the Bush administration.
The Pentagon has previously said Khalid Sheikh Mohammed admitted he was responsible "from A to Z" for the 9/11 attacks.
In a previous court hearing he said that he intended to plead guilty and would welcome martyrdom.
He was captured in Pakistan in March 2003 and has been detained at Guantanamo Bay since 2006.
US prosecutors accuse him of involvement with the 2002 nightclub bombing in Bali, the 1993 World Trade Center bombing, the murder of American journalist Daniel Pearl and a failed attempt in 2001 to blow up an airliner using a shoe bomb.