The US has reached a plea deal with three of the men behind the September 11 attacks, the Pentagon has announced.
Khalid Sheikh Mohammed, the man accused of masterminding the 2001 al Qaeda attacks, and two of his accomplices, held at the US military prison at Guantanamo Bay in Cuba, have agreed to plead guilty.
The Pentagon did not immediately elaborate on the terms of the plea bargain.
A US official, speaking on condition of anonymity, said the plea deals almost certainly involved guilty pleas in exchange for taking the death penalty off the table.
Mohammed and his accomplices, Walid Bin Attash and Mustafa al-Hawsawi, are expected to enter the pleas at the military commission at Guantanamo Bay as soon as next week.
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 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 Qaeda's attack, and over 20 years after militants crashed two hijacked airliners into the World Trade Centre and brought down the twin towers.
The attack, which included a passenger plane flying into the Pentagon, killed nearly 3,000 people and triggered years of US wars against militant extremist groups that reshaped Middle Eastern countries.
Responding to news of the plea deal Terry Strada, national chairwoman of 9/11 Families United, a group of families of victims, said many families just wanted to see the men admit guilt.
"For me personally, I wanted to see a trial," she said. "And they just took away the justice I was expecting, a trial and the punishment.
"They were cowards when they planned the attack. And they're cowards today."
Dozens of relatives of those killed have died while awaiting resolution of the case, Ms Strada added.
Authorities captured Mohammed in 2003. Mohammed was subjected to waterboarding 183 times while in CIA custody before being taken to Guantanamo, and was also subjected to other torture and coercive questioning.
Mohammed is the most well known inmate at the detention facility in Guantanamo Bay, which was set up in 2002 by then-US President George W Bush to house foreign militant suspects following the September 11 attacks on the United States.
Its population grew to a peak of about 800 inmates before it started to shrink. There are 30 inmates today.
The use of torture has proven one of the most formidable obstacles in US efforts to try the men in the military commission at Guantanamo, owing to the inadmissibility of evidence linked to abuse.
Torture has accounted for much of the delay of the proceedings, along with the courtroom's location a plane ride away from the United States.
Michael Burke, one of the family members receiving the government notice of the plea bargain, condemned the long wait for justice, and the outcome.
"It took months or a year at the Nuremberg trials," said Mr Burke, whose fire captain brother Billy died in the collapse of the World Trade Centre's North Tower.
"To me, it always been disgraceful that these guys, 23 years later, have not been convicted and punished for their attacks, or the crime. I never understood how it took so long.
"I think people would be shocked if you could go back in time and tell the people who just watched the towers go down, 'Oh, hey, in 23 years, these guys who are responsible for this crime we just witnessed are going to be getting plea deals so they can avoid death and serve life in prison'."
US Senate Republican Leader Mitch McConnell condemned the plea deals.
"The only thing worse than negotiating with terrorists is negotiating with them after they are in custody," McConnell said in a statement, accusing the administration of Democratic President Joe Biden of "cowardice in the face of terror."
