The Pentagon on Monday sought murder and conspiracy charges against the alleged planner of the September 11 attacks, Khalid Sheikh Mohammed, and five others and will ask they be executed if convicted.
The charges, if approved by a Pentagon appointee who oversees the war court at Guantanamo, are the first from that court alleging direct involvement in the 2001 attacks on the United States and the first involving the death penalty.
Suspects were also charged with terrorism and violating the laws of war and targeting civilians.
"The defendants will face the possibility of being sentenced to death," Air Force Brig Gen Thomas Hartmann told reporters.
Mohammed, a Pakistani national better known as KSM, has said he planned every aspect of the September 11 attacks. But his confession could be problematic if used as evidence because the CIA has admitted it subjected him to "waterboarding," a simulated drowning technique.
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 United States has signed.
The charges against Mohammed will include conspiring with Al-Qaeda to attack and murder civilians and about 3,000 counts of murder for those killed in the September 11 hijacked plane attacks.
Mohammed also said he was responsible for a 1993 attack on New York's World Trade Center, the bombing of a nightclub in Bali, Indonesia, and an attempt to down two US airplanes using shoe bombs. He also confessed to the beheading of US journalist Daniel Pearl.
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.
The US military began sending captives to Guantanamo, a US base on the southeast tip of Cuba, in January 2002 and hopes to eventually try 80 of the 275 who remain.
The widely criticized Guantanamo tribunals are the first US war crimes tribunals since World War II.
They were established after the September 11 attacks to try non-US captives whom the Bush administration considers "enemy combatants" not entitled to the legal protections granted to soldiers and civilians.
They currently operate under authority of a law Congress passed in 2006, after the US Supreme Court struck down the first version.
(Editing by Patricia Wilson and Frances Kerry)