Pakistani national Majid Khan has pleaded guilty at a Guantanamo military tribunal in a landmark case that could speed the trials of September 11 suspects.
Majid Khan, 32, a protege of September 11 mastermind Khalid Sheikh Mohammed, pleaded guilty to conspiracy, murder and attempted murder in violation of the laws of war, and to material support for terrorism and espionage.
Dressed in a dark suit and pink tie, he spoke in English without an interpreter in delivering his plea.
Khan, who has spent the last nine years behind bars, faced possible life in prison but is expected to receive a reduced sentence as part of a plea agreement.
In exchange for the lighter sentence, he will testify against other "high value" detainees, including Mohammed and four others alleged to have taken part in the 2001 attacks.
Many of the terms of the plea agreement remain classified. The Washington Post reported that the military plans to delay Khan's sentencing for four years to ensure he complies with the agreement.
"It's part of a strategy of building more solid cases against the handful of defendants that the government plans to try before the commissions," said Jonathan Hafetz, a lawyer who has represented other Guantanamo detainees.
More than 10 years after the September 11 attacks, Mohammed and four co-defendants accused of plotting them are still awaiting trial at the prison, part of a US naval base in Cuba.