The man accused of being the main plotter in al Qaida’s September 11 attacks in 2001 has agreed to plead guilty, the US Defence Department said.

Khalid Sheikh Mohammed and two accomplices, Walid Bin Attash and Mustafa al-Hawsawi, are expected to enter the pleas at the military commission at Guantanamo Bay, Cuba, as soon as next week.

Pentagon officials declined to immediately release the terms of the plea bargain. The New York Times, citing unidentified Pentagon officials, said the terms included the men’s longstanding condition that they be spared risk of the death penalty.

Defence lawyers have requested the men receive life sentences in exchange for the guilty pleas, according to letters from the federal government received by relatives of some of the nearly 3,000 people killed outright on the morning of September 11.

The US agreement with the men to enter into a plea agreement comes more than 16 years after their prosecution began for al Qaida’s attack, and over 20 years after militants flew commandeered commercial airliners into buildings.

The attack killed nearly 3,000 people and triggered years of US wars against militant extremist groups that reshaped Middle Eastern countries.

Authorities captured Mohammed in 2003. Mohammed was subjected to waterboarding 183 times while in CIA custody before coming to Guantanamo, along with other torture and coercive questioning.