The defendant behind the September 11, 2001 attacks is scheduled to plead guilty on Friday, more than 23 years after the death of nearly 3.000 people in the United States shocked the world.
Khalid Sheikh Muhammed, often referred to only as KSM, is scheduled to appear in a court-martial at the naval base at Guantanamo Bay, in southeastern Cuba, where he has been held in a military prison for nearly two decades.
Muhammad is the most notorious Guantanamo prisoner and one of the last to be held at the base.
But there could be further delays as the US government has argued that allowing a settlement to take place would cause "irreparable" harm to both it and the public.
A day before plea deals are due to be heard, there is confusion among families, officials and legal teams at the base as they wait to see what happens next.
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What is scheduled this week?
In a hearing scheduled to begin Friday morning, Muhammad is expected to plead guilty to his role in the Sept. 11, 2001, attacks, when hijackers seized airliners and crashed them into the World Trade Center in New York and the Pentagon near Washington.
Another plane crashed into a field in Pennsylvania after passengers resisted the hijackers.
Muhammad was charged with crimes including conspiracy and murder, and his indictment listed the names of all 2.976 victims.
He has previously said he planned "Operation 9/11 from A to Z" - coming up with the idea of training pilots to crash commercial airliners into buildings and pitching those plans to Osama bin Laden, the leader of the militant Islamist group Al Qaeda, in the mid-XNUMXs.
If it comes to it, Friday's hearing is scheduled to take place in a courtroom at the base, where families of the victims and journalists will sit in an observation gallery behind thick glass.
Why is this happening 23 years after the attack?
The trial, held in a military court at a naval base, has dragged on for more than a decade, further complicated by questions over whether the torture of Muhammad and the other defendants while in US custody tainted evidence.
After his arrest in Pakistan in 2003, Muhammad spent three years in secret CIA prisons known as "black sites," where he was subjected to simulated strangulation, or "waterboarding," 183 times, among other so-called "advanced interrogation techniques" in which include sleep deprivation and forced nudity.
Karen Greenberg, author of The Least Worst Place: How Guantanamo Became the World's Most Infamous Prison, says the use of torture has made it "virtually impossible to bring these cases to court in a way that respects the rule of law and American jurisprudence."
"It is obviously impossible to present evidence in these cases without using evidence obtained through torture.
"Furthermore, the fact that these individuals were tortured adds another layer of complexity to the prosecution itself," she says.
The case also falls under the jurisdiction of military commissions, which operate under different rules than traditional American criminal justice, which slows down the process itself.
The settlement was reached last summer, after two years of negotiations.
What does this settlement entail?
All the details of the settlement reached with Muhammad and two of his co-defendants have not been released.
We know that the plea deal means he will not face a death penalty trial.
At Wednesday's hearing, his legal team confirmed that he had agreed to plead guilty to all charges.
Muhammad did not address the court personally, but spoke to his own team who went through the entire settlement, making small corrections and wording changes together with the prosecution and the judge.
If he pleads guilty, and the court accepts the plea deal, the next step will be the appointment of a military jury, known as a panel, which will hear evidence during the actual sentencing.
In court on Wednesday, lawyers described it as a kind of public trial, where survivors and family members of the victims would have a chance to give their own statements.
Key to the prosecution's agreement to the settlement was a guarantee "that we could present all the evidence we deemed necessary to establish a historical record of the defendants' involvement in what happened on September 11," said prosecutor Clayton J. Trivett Jr.
But even if the settlements are approved, it will be many months before these proceedings begin and eventually a verdict is handed down.
Why is the US government trying to block settlements?
US Defense Secretary Lloyd Austin named the senior official who signed the settlement.
But he was on the road at the time of the signing and was reportedly taken aback by the act, according to the New York Times.
A few days later, he tried to withdraw it, saying in a letter: "The responsibility for such a decision should be on me as the supreme authority."
However, both the military judge and the military appeals board ruled that the bargain was valid and that Austin acted too late.
In another attempt to block the settlement, the government this week asked a federal appeals court to intervene.
The court filing states that Muhammad and the other two are accused of "committing the most serious crime on American soil in its modern history" and that implementing the settlement would "deprive the government and the American people of a public trial to determine the defendant's guilt and the possibility of the death penalty, despite the fact that the Minister of Defense withdrew those agreements".
After the settlement was announced last summer, Republican Sen. Mitch McConnell, then the House leader, issued a statement describing it as a "heinous abdication of the government's responsibility to defend America and ensure justice."
What do the families of the victims say?
Some families of the victims of the attacks also criticized the settlement, saying it was too lenient or non-transparent.
Speaking to the BBC last summer, Teri Strada, whose husband Tom was killed in the attacks, described the settlement as "giving the prisoners at Guantanamo exactly what they want".
Strada, the national president of the 9/11 United Families activist group, said: "This is a victory for Khalid Sheikh Mohammed and the other two, this is a victory for them."
Other families see the agreements as a path to judgments in a complex and lengthy process and were disappointed by the government's latest intervention.
Steven Gerhart, whose younger brother Ralf was killed in the attacks, flew to Guantanamo Bay to watch Muhammad's plea.
"What is the ultimate goal of the Biden administration? To get a reprieve and to transfer everything to the next administration? With what goal? Think of families. Why are you prolonging this saga?” he said.
Gerhart told the BBC that the settlements were "not a victory" for the families, but that it was "time to find a way to wrap this up, to bring these people to justice."
Why is the procedure being conducted in Guantanamo?
Muhammad has been held in the military prison at Guantanamo Bay since 2006.
The prison opened 23 years ago - on January 11, 2002 - during the "war on terror" that followed the 9/11 attacks, as a place to hold suspected terrorists and "illegal enemy combatants".
Most of those held there have never been charged, and the military prison has come under fire from human rights groups and the United Nations for its treatment of detainees.
Most have now been repatriated or moved to other countries.
There are currently 15 of them in the prison - which is the lowest number in its history.
All but six of them were accused or convicted of war crimes.
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