The self-confessed mastermind of the 9/11 attacks and four co-accused will be formally arraigned on Saturday at Guantanamo Bay, a last step before a long-awaited trial after years of legal delays.

Before a military commission, Khalid Sheikh Mohammed and the four others will be charged with planning and executing the September 11, 2001 attacks that left 2,976 people dead in New York, Washington and Shanksville, Pennsylvania.

It will mark a decisive stage more than a decade after the most lethal attacks on US soil in modern history. It also comes the same week that President Barack Obama is marking a year since he ordered the US Navy SEALs raid that took down the man behind it all - Osama bin Laden.

Mohammed, known as KSM, faces the death penalty for his crimes, along with Ramzi Binalshibh of Yemen, KSM's Pakistani nephew Ali Abd al-Aziz Ali - also known as Ammar Al-Baluchi - Walid bin Attash and Mustapha al-Hawsawi of Saudi Arabia.

"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 enhanced interrogation techniques on terror suspects.

One of the last steps before the so-called 'trial of the century' takes place, the arraignment has generated feverish interest. It marks the second time the United States has tried to prosecute the 9/11 suspects.

Out of 200 applicants, 60 journalists have obtained a seat 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, said.

Pentagon spokesperson Lieutenant Colonel Todd Breasseale said that since the special military tribunals resumed last year after being reformed by the Obama administration - to better comply with legal standards - the number of journalists covering the proceedings has 'more than tripled'.

It has been nine years since Mohammed's 2003 arrest, three of which he spent in secret CIA jails, confessing to series of attacks and plots after being subjected to harsh interrogations, including waterboarding, denounced by rights groups as torture.

A Kuwaiti of Pakistani origin, he could take advantage of having a public at the proceedings to "deliver a diatribe against the US government," analyst David Rivkin said.

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 on a giant screen.

But Terry Greene of the September Eleventh Families for Peaceful Tomorrow, a group of relatives of 9/11 victims, said she would have favored a civil trial in federal court that would have been "more accessible to family members."

Greene, whose brother Donald Freeman Greene was among the passengers aboard the hijacked United Flight 93 that crashed in Shanksville, expressed 'great disappointment' at the decision to hold the case under the military commissions system crafted by Bush.

"It would have been better to have a speedy trial rather than wait for years and years," she said.

Obama, a Democrat, wanted to hold the trial in civil court in Manhattan, just steps from Ground Zero, where the Twin Towers once stood. But stiff Republican opposition in Congress scuttled those plans.

"The whole system is unfair," said Ali's attorney James Connell.

Defense lawyers have planned to challenge a rule that keeps statements by Guantanamo detainees secret, and hampered access to classified information.

The time that has lapsed since the attacks and the arrest of their presumed authors is another concern likely to come up at the hearing, said Connell.

"It is fair to anticipate that we will raise the delay as an issue in the case," he said.

But the trial itself could still be years away, unless Mohammed pleads guilty to be put to death sooner and become a 'martyr' for Al-Qaeda.