The US Department of Defense (Pentagon) said on Wednesday that the accused mastermind of the September 11 attacks, Khalid Sheikh Mohammed, and two other defendants held at the US military prison in Guantanamo Bay, Cuba, have agreed to "plead guilty."

The Pentagon did not provide details of the plea agreements.

A US official, speaking on condition of anonymity, said the agreements certainly included a guilty plea in exchange for not imposing the death penalty.

The official said the terms of the deal had not been made public, but he acknowledged the possibility of a life sentence.

Khalid Sheikh Mohammed is the most notorious prisoner at the facility at Guantanamo Bay, which was established by then-President George W. Bush in 2002 to hold foreign suspects in the wake of the September 11, 2001, attacks on the United States.

The prison population rose to a peak of about 800 prisoners, before declining again, and there are now 30 prisoners.

Khalid Sheikh Mohammed is accused of plotting to hijack commercial airliners and crash them into the World Trade Center in New York City and the Pentagon.

The September 11 attacks killed nearly 3,000 people and pushed the United States into what would become a nearly 20-year war in Afghanistan.

His interrogations have long been the subject of scrutiny. A 2014 Senate Intelligence Committee report on the CIA's use of waterboarding and other interrogation techniques said Mohammed was waterboarded at least 183 times.

The Pentagon statement said that the three were initially charged with joint charges and tried on June 5, 2008, then charged with joint charges again and tried for a second time on May 5, 2012.

Senate Republican Leader Mitch McConnell denounced the plea agreements.

“The only thing worse than negotiating with terrorists is negotiating with them after they’ve been captured,” McConnell said in a statement, accusing the administration of Democratic President Joe Biden of “cowardice in the face of terrorism.”