An alleged mastermind of the September 11, 2001, attacks and four other accused co-conspirators are to be formally charged at a military tribunal in Guantanamo Bay.
Khalid Sheikh Mohammed and his co-defendants are to be arraigned on Saturday on charges that include 2,976 counts of murder, one for each person killed after hijacked planes struck New York, Washington and Pennsylvania.
They will appear in public for the first time in more than three years, when US officials start a second attempt at what is likely to be a drawn out legal battle that could lead to the men's executions.
During the failed first effort to prosecute them at the US base located in Cuba, Mohammed had mocked the tribunal and said he and his co-defendants would plead guilty and welcome 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 would be "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," Connell 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 defendants 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.
Khalid Sheikh Mohammed is a Pakistani who grew up in Kuwait. Captured in 2003, he was transferred to Guantanamo Bay in 2006.
Saudi national Mustafa al-Hasawi is accused of being the financier of the 9/11 attacks.
Walid bin Attash is a Yemeni who grew up in Saudi Arabia. The US suspects he was a bodyguard and chief lieutenant to Osama bin Laden, the late leader of al-Qaeda.
Ramzi bin al-Shibh, a Yemeni who the US says would have been the 20th hijacker in the 9/11 attacks, was captured nearly 10 years ago and has been held at Guantanamo since 2006.
Finally, Ammar al-Baluchi, also known as Ali Abdul Aziz Ali, is alleged by the US to have played a role in the planning, funding and logistics of 9/11.
Jim Harrington, a civilian lawyer for Shibh, said he did not think any of the defendants would plead guilty, notwithstanding their earlier statements.
Army Captain Jason Wright, one of Mohammed's Pentagon-appointed lawyers, declined to comment on the case.
'Not fair trial'
"I don’t think it is going to be a fair trial, as a fair trial has lot of elements which have to be included in the trial itself," Najeeb al-Nuaimi, a former Qatari minister of justice, told Al Jazeera.
"But what we know from procedures of the trial is that the defender will be deprived of many elements, and information of the investigation will be classified. Though their lawyers have been shown [the information], they cannot speak about it," he said.
"A special court is not recognised by international practice. A special court is just like having military court in Bahrain, Syria or Egypt, and it has not been recognised by human rights organisations, not even by the UN... This doesn’t create justice."
As in previous hearings, a handful of people who lost family members in the attacks have been selected by lottery to travel to the base to watch the proceedings.
Several said they were grateful for the chance to see a case they believe had been delayed too long.
Cliff Russell, whose firefighter brother Stephen died responding to the attack on the World Trade Center in New York, said he hoped the case would end with the death penalty for the five Guantanamo prisoners.
The arraignment for the five comes more than three years after President Barack Obama's failed effort to try the suspects in a federal civilian court and close the Guantanamo base.
Congress blocked the transfer of any prisoners from Guantanamo to the US, forcing the Obama administration to refile the charges under a reformed military commission system.
New rules adopted by Congress and Obama forbid the use of testimony obtained through cruel treatment or torture.
But human rights groups and defence lawyers say the reforms have not gone far enough and that restrictions on legal mail, and the overall secret nature of Guantanamo and the commissions, make it impossible to provide an adequate defence.