The man who called himself the "mastermind" of the September 11 attacks, and four comrades, are back before a military judge in Guantanamo Bay to face charges that could lead to their execution.
Khalid Sheikh Mohammed and his co-defendants appeared under heavy security at the US base in Cuba. The five face charges that include terrorism and 2,976 counts of murder each for their alleged roles planning and aiding the September 11 attacks.
The hearing is the first time the five have been in court in nearly three and a half years.
President Barack Obama put their previous tribunal on hold in a failed effort to move the case to a civilian court.
Mohammed has mocked the tribunal and said in previous court appearances that he welcomed execution.
But there were signs that at least some of the defence teams were preparing for a lengthy fight, planning challenges of the military tribunals and the secrecy that shrouds the case.
The arraignment is "only the beginning of a trial that will take years to complete, followed by years of appellate review", lawyer James Connell, who represents defendant Ali Abd al-Aziz Ali, told reporters gathered at the base to observe the hearing. "I can't imagine any scenario where this thing gets wrapped up in six months," he said.
Defendants in what is known as a military commission typically do not enter a plea during their arraignment.
Instead, the judge reads the charges, makes sure the accused understand their rights and then moves on to procedural issues. Lawyers for the men said they were prohibited by secrecy rules from disclosing the intentions of their clients.
Jim Harrington, a civilian lawyer for Ramzi Binalshibh, a Yemeni prisoner who has said at one hearing that he was proud of the September 11 attacks, said he did not think that any of the defendants would plead guilty, notwithstanding their earlier statements. Army Capt Jason Wright, one of Mohammed's Pentagon-appointed lawyers, declined to comment on the case.