All five men were called before combat status review tribunals two years ago at the Guantánamo detention camp.
Transcripts of the tribunals offer a rough preview of the kind of case that may emerge against each of the five terror suspects.
What follows is a summary of the allegations and known evidence for each of the five prospective defendants:
Khalid Sheikh Mohammed
The self-described mastermind of the 9/11 attacks, offered a complete confession in a combat status review tribunal conducted at Guantánamo Bay in 2007. It is unclear whether this confession will be admitted as evidence in his trial. It may offer a preview of Mr. Mohammed's trial strategy – to admit involvement in a wide range of terror attacks and die a martyr after being convicted and sentenced to death.
"I was responsible for the 9/11 operation, from A to Z," he told the military tribunal in March 2007.
He admitted to swearing an oath of allegiance to Osama Bin Laden to conduct jihad and to serving Bin Laden as operational director for the "organizing, planning, follow-up, and execution of the 9/11 operation."
Mohammed was subject to some of the harshest interrogation tactics used by US officials. He was reportedly waterboarded 183 times. In his 2007 statement, he complained bitterly about being tortured by the CIA, but it didn't seem to temper his apparent desire to confess.
He claimed responsibility for the 1993 World Trade Center bombing, the decapitation of Wall Street Journal reporter Daniel Pearl, Richard Reid's failed shoe bomber operation, the nightclub bombing in Bali, Indonesia, planning for a second-wave of attacks on the US allegedly targeting the Liberty Tower in Los Angeles, the Sears Tower in Chicago, the Plaza Bank in Seattle, and the Empire State Building in New York, an assassination plot against former President Carter and former President Clinton, a plot to bomb bridges in New York, a plot to attack Heathrow Airport and Canary Wharf, plots to bomb US embassies in Indonesia, Australia, and Japan, the launch of a Russian-made SA-7 missile at a commercial jet departing Mombasa, a plan to hit nuclear power plants in the US, a plan to destroy 12 US commercial jets in flight at the same time, and a plot to train and fund an assassination attempt against former Pakistan President Pervez Musharaf.
Walid Bin Attash
He is a Yemeni who was born and raised in Saudi Arabia. Mr. Attash lost his right leg on the battlefield in Afghanistan and was sometimes referred to by the code name "Father of the Leg." He allegedly served Al-Qaeda as an intermediary between Osama Bin Laden and Abd al-Rahim al-Nashiri, the accused leader of the plot to attack the USS Cole off the coast of Yemen.
Bin Laden is believed to have tapped Mr. Attash to serve as one of the hijackers in the 9/11 attacks, but he was detained in Yemen in April 2001 while attempting to obtain a US visa. Earlier in 1999, he helped select operatives for special training in Afghanistan. They included a recruit who later became a suicide bomber in the Cole attack, and two of the 9/11 hijackers.
Attash reportedly traveled to Kuala Lumpur and Bangkok in December 1999 and January 2000 to meet with two of the 9/11 hijackers. He also is said to have flown two US-owned airliners to observe security procedures.
During his March 2007 combat status review tribunal at Guantánamo, Attash was asked his role in the USS Cole attack.
"Many roles," Attash replied, according to the transcript. "I participated in the buying or purchasing of the explosives. I put together the plan for the operation a year and a half prior to the operation. Buying the boat and recruiting the members that did the operation."
The suicide attack in Yemen's port of Aden killed 17 US sailors and wounded 39.
Ramzi Bin al-Shibh
He was born in 1972 in southern Yemen. Al-Shibh was allegedly selected to participate in the 9/11 hijacking. He and three of the hijackers, including leader Mohammed Atta, met in Hamburg, Germany, in the late 1990s. They traveled to Afghanistan in 1999 where they allegedly met Osama Bin Laden, pledged their loyalty, and agreed to participate in a martyrdom mission against the US.
Al-Shibh was described by a senior Al-Qaeda military commander as a "highly professional jihadist," according to court documents. His Al-Qaeda instructions were reportedly to enroll in a flight training school in Florida where hijacker Ziad Jarrah was already attending classes. Al-Shibh paid a $2,350 deposit to the school. But he never got there.
After receiving a new passport in Germany, Al-Shibh tried four times but was unsuccessful in obtaining a US visa.
Instead of being a hijacker, Al-Shibh served the 9/11 operation as a key intermediary between the hijackers in the US and Al-Qaeda leaders in Pakistan and Afghanistan, according to a US report. He allegedly relayed instructions by email and phone and met with Atta in Spain in July 2001 to obtain a final briefing on the progress of the operation. He is alleged to have coordinated travel plans for some of the hijackers and helped transfer money to them.
He is reported to have sent $15,000 to Zacharias Moussaoui, who was then undergoing flight training in the US.
In a June 2002 interview with the television station Al-Jazeera, Al-Shibh and Khalid Sheikh Mohammed explained how the 9/11 attacks were planned and carried out. In the interview, Mohammed described Al-Shibh as a coordinator of the 9/11 attacks.
Ali Abdul-Aziz Ali
He is Khalid Sheikh Mohammed's nephew. Described as a close aide to his uncle, Ali set up bank accounts in the United Arab Emirates and allegedly helped transfer more than $100,000 to the hijackers in the US. He also allegedly helped facilitate the hijackers travel from Pakistan to the US via the UAE.
A witness told the FBI that during his time in the UAE Ali was in constant contact with his uncle.
When he appeared at his combat status review tribunal in March 2007, Ali admitted transferring money to one of the hijackers, but he denied any knowledge that an Al-Qaeda operation was underway.
"When Marwan al-Shehhi approached me, he never declared himself as 'hijacker Marwan al-Shehhi.' He approached me the same way he approached other individuals and companies in the US – a man wanting to do business," Ali said, according to a transcript of the hearing.
He said he was asked by his uncle to conduct a series of business transactions. He assumed the participants were businessmen. Ali said he had no knowledge of Al-Qaeda or his uncle's involvement with Al-Qaeda.
"He never mentioned it to me," Ali said.
Ali told the tribunal he had never traveled to Afghanistan and never met Osama Bin Laden.
He said he was tricked into assisting Al-Qaeda through his own ignorance in the same way that US banks, flight schools, rental car companies, and property managers were tricked.
Ali's 2007 tribunal accepted a written statement from Ali's uncle Mohammed. "Any dealings he had with Al-Qaeda were through me," Mohammed said in the statement. "I used him for business dealings. He had no knowledge of any Al-Qaeda links."
In his statement, Mohammed took responsibility for the money transfers himself.
"I moved money to Mohammed Atta and Ramzi [Bin al-Shibh] in Germany to support the 9/11 attacks," Mohammed said. "I don't remember using [Ali] for the 9/11 hijackers. I used mostly Hawsawi in 2000 and 2001."
Mustafa Ahmad al-Hawsawi
He is believe to be one of Al-Qaeda's money men for the 9/11 operation. He allegedly shared a United Arab Emirates-based bank account with one of the hijackers. This provided living expenses for the hijackers immediately prior to the terror attacks. Two of the hijackers returned $18,000 in unused funds directly to al-Hawsawi in the last days before the attacks. He allegedly followed Khalid Sheikh Mohammed's instructions and wired thousands of dollars to Bin al-Shibh in the summer of 2001.
At the time of his capture in Pakistan, Hawsawi was in possession of a laptop computer containing Al-Qaeda expense reports, family information for Al-Qaeda operatives, Al-Qaeda casualty lists, and family allowance information.
In a March 2007 appearance before his combat status review tribunal at Guantánamo, Hawsawi denied knowledge of many of the computer files. He said several individuals in the Al-Qaeda safe house were downloading information into the same computer.
But he also admitted receiving money from hijackers Mohammed Atta and Waleed Al-Shehri shortly before the 9/11 attacks. He said he first learned of the 9/11 operation on that day. He said he was told by Ramzi Bin al-Shibh and Khalid Sheikh Mohammed to return immediately to Pakistan from the United Arab Emirates.
Hawsawi admitted meeting four of the hijackers in the UAE and communicating with Atta. But he denied transferring money to them.
He said he had met Osama Bin Laden four or five times but had never taken a loyalty oath. He insisted he was not a member of Al-Qaeda.