The self-confessed mastermind of the 9/11 attacks and four co-accused were due to be arraigned Saturday at Guantanamo Bay, with all facing the death penalty if convicted.
Khalid Sheikh Mohammed and his co-defendants will be formally charged in a military tribunal with planning and executing the September 11, 2001, attacks that killed 2,976 people in New York, Washington and Shanksville, Pennsylvania.
One of the last steps before the so-called "trial of the century" takes place, the arraignment marks the second time the United States has tried to prosecute the 9/11 suspects.
It comes more than a decade after the most lethal attacks on US soil in modern history, and just about one year after President Barack Obama ordered the US Navy SEALs raid that killed the man behind it all -- Osama bin Laden.
"There is a desire for justice. It is an important moment for all of us," said Marc Thiessen, a former speechwriter for president George W. Bush who has defended the Bush administration's use of what it called "enhanced interrogation techniques" on terror suspects -- which rights groups have denounced as torture.
Mohammed will appear in the military tribunal along with Ramzi Binalshibh of Yemen, Mohammed's Pakistani nephew Ali Abd al-Aziz Ali -- also known as Ammar al-Baluchi -- Walid bin Attash and Mustapha al-Hawsawi of Saudi Arabia.
The five have been held for years at the US-run prison in Guantanamo Bay, Cuba, while a legal and political battle has played out over how and where to prosecute them -- and debates have raged over how they were treated.
It has been nine years since Mohammed's 2003 arrest, three of which he spent in secret CIA jails, confessing to a series of attacks and plots after being subjected to harsh interrogations, including waterboarding.
Some analysts say Mohammed could try to take advantage of the intense interest in the proceedings to deliver a scathing attack on the US government.
Out of 200 applicants, 60 journalists have obtained seats for the hearing at the US naval base in southern Cuba, while another 30 will cover the event from Fort Meade in Maryland using a closed-circuit television feed.
"It's key to have transparency," the military commissions' chief prosecutor, Brigadier General Mark Martins, told AFP.
In a sign of the acute public interest in the proceedings, the Pentagon has opened four military bases on US territory to allow families of the 9/11 victims to watch the case unfold on a giant screen.
Obama, a Democrat, wanted to hold the trial in a civil court in Manhattan, just steps from Ground Zero, where the World Trade Center's Twin Towers once stood. But stiff Republican opposition in Congress scuttled those plans.
The military tribunal system however has come under scrutiny, and some of the sharpest criticism has come from former chief prosecutor Morris Davis, who told AFP: "History will judge this as a mistake."
The time that has lapsed since the attacks and the arrest of their presumed authors is a concern likely to come up at the hearing, said Ali's attorney James Connell.
"It is fair to anticipate that we will raise the delay as an issue in the case," he said.
Davis agreed the long delays were a problem, saying: "The fact that KSM has not been pushed to trial earlier is a testament to the failure of the military commissions system."
The trial could still be years away, unless Mohammed pleads guilty to be put to death sooner and become a "martyr" for Al-Qaeda.