He has previously chosen to brag about his role as the mastermind of the worst terrorist atrocity in history.
But on Saturday Khalid Sheikh Mohammed sat mutely in his chair in a courtroom in the Guantanamo US naval base and refused to respond to a judge's questions at the arraignment of five men accused of the September 11 terrorist attacks.
The co-accused staged a show of silent defiance in a rocky start to the first court hearing of what is being described as the "trial of the century".
Walid bin Attash initially sat slumped in his chair in seat-belt type restraints after resisting attempt to bring him to court from his cell. He was later released from the shackles, on telling his lawyer that he would not disrupt the hearing.
And proceedings at the military tribunal were halted when a third defendant, Ramzi bin al-Shibh, went through a series of elaborate prayer rituals, kneeling down next to his desk on a mat.
It was the first public appearance in more than three years for Mohammed, the self-confessed architect of the worst act of terrorism the world has witnessed, and his four alleged co-conspirators. The five men face the death penalty for 2,976 individual counts of murder.
Mohammed stroked his straggly thick beard, grey when last seen, but now tinged heavily reddish-brown with henna, yet showed no emotion as he stared ahead blankly or looked down at notes on his desk. He was wearing white robes, issued by the military authorities, and a white cap.
The five removed earphones carrying Arabic translation and indicated they would not co-operate with the proceedings, to the exasperation of Colonel James Pohl, the military judge. He also scolded defence lawyers who attempted to raise the men's complaints about their treatment at the Guantanamo detention centre.
Much of the first hour was bogged down in arguments about translation. Even the simultaneous interpreters complained at one stage about being asked to translate when the judge and defence lawyers were speaking at the same time.
The five men ignored the judge as he asked them if they accepted their defence team - a combination of military lawyers and civilian attorneys who specialise in fighting death penalty cases.
"Accused refuses to answer," Col Pohl said repeatedly for the court record after being met with silence.
He was expected later to ask the defence lawyers if they wished to waive the right to have read the full 123-page charge sheet - including more than 60 pages listing the names of the dead.
He was also due to ask the defendants whether they wanted to plead guilty, not guilty or to reserve their plea. But the legal stalemate over procedural issues and motions of complaint by the defence could mean that phase would be held over until another hearing.
The defence lawyers were turned down in their requests to raise the men's complaints about their treatment, and that they were prevented from wearing civilian clothes of their choice.
"I believe Mohammed will decline to address the court. I believe he's deeply concerned about the fairness of the proceeding," said his civilian lawyer, David Nevin.
"The reason that he is not putting the earphones on is because of the torture that was inflicted upon him."
Lawyers have previously been told that they cannot discuss the accusations of torture or the activities of the CIA, whose agents interrogated Mohammed using waterboarding - simulated drowning - 183 times, according to a US federal report.
Col Pohl warned that he would not permit defendants to block the hearing and would continue without Mohammed's participation. "One cannot choose not to participate and frustrate the normal course of business," he said.
The start of the hearing was delayed as bin Attash had to be forced by guards from his cell to be led through a network of corridors to the specially constructed courtroom in a hangar-style structure.
Cheryl Bormann, a civilian attorney for bin Attash, wore a black hijab and long black robe and told the court that the treatment of her client at Guantanamo had interfered with his ability to participate in the proceedings. "These men have been mistreated," she said.
She asked the judge to order other women in court to cover themselves up, saying it was wrong that otherwise the defendants would be "forced to not look at the prosecution for fear of committing a sin under their faith".
The judge said that until the question of the men's legal representation was settled, the attorneys had no standing to make motions concerning the defendants' treatment. But the defendants still refused to answer the judge's questions.
The only one of the accused to speak during the morning session was al-Shibh, who interrupted a testy exchange between the judge and a lawyer with an outburst about Muammar Gaddafi, the former Libyan dictator. When Col Pohl told him to be quiet, he continued in broken English: "Maybe they will kill me and say I committed suicide. Maybe you are not going to see me any more. This is the way that we are treated in this camp."
The otherwise silent protest was in marked contrast to the last time that the defendants appeared in court in the long-stalled legal process in December 2008 when Mohammed tried to confess and plead guilty and declared his desire to be a martyr.
He revelled in his role as the September 11 ringleader, boasting that he was responsible for the attacks "from A to Z" and signing court documents as "KSM", the initials by which US law enforcement referred to him during their global manhunt.
The charge sheet listed his meticulous eye for control and man management. With his US college education, he coached the Arabic hijackers for their mission with English phrases such as "If anyone moves, I'll kill you", then oversaw their practice for cutting the throats of flight crew with craft knives by slaughtering sheep and goats.
The attacks that the five are accused of planning, organising and financing struck at the heart of America. They destroyed the twin towers of the World Trade Centre in New York, the symbol of the country's financial strength; hit the Pentagon, its military hub; and were only thwarted in their plan to attack its political heart - the White House or more likely Congress's offices - when passengers brought down Flight 93 in a Pennsylvania field.
For the relatives of those killed on September 11, it was a bittersweet day as - more than a decade since the attacks and nine years since Mohammed was captured in Pakistan - the final steps toward justice for the accused began.
Five family members attended the hearing in Guantanamo; dozens of others watched the proceedings on a live video-stream at four US military bases in the north-east of the US, including Fort Meade, a sprawling army installation in Maryland that is home to the US Cyber Command and the National Security Agency.
Relatives at Guantanamo said they were grateful for the chance to see a case they believe has been delayed too long.
Cliff Russell, whose brother Stephen, a fireman, died at the twin towers, said he hoped the case would end with the death penalty.
"I'm not looking forward to ending someone else's life and taking satisfaction in it," he said. "But it's the most disgusting, hateful, awful thing I ever could think of if you think about what was perpetrated."
Suzanne Sisolak of Brooklyn, whose husband Joseph was killed in his office in the World Trade Centre's North Tower, said she was not concerned about the outcome as long as the case moved forward. "They can put them in prison for life. They can execute them," she said. "What I do care about is that this does not happen again."
Maureen Basnicki, 61, whose husband Ken was attending a breakfast conference in the Windows on the World restaurant on the 106th floor of the North Tower, flew in from Canada to follow the hearing at the base cinema in Fort Hamilton in Brooklyn.
"Part of me was trying to get a sense of justice and to understand why these men were so full of hate that they would slaughter so many innocent civilians, though I always doubted those questions would be answered for me," she said
"But what was really important for me was to share this moment with other 9/11 family members. It is their support and solidarity that matters to me at this time. I wanted to be with them this day."
Alongside her was her friend, Debra Burlingame, whose brother Charles was pilot of the hijacked American Airlines Flight 77 that was flown into the Pentagon.
"This is the start of the process of bringing justice for my brother, his crew and passengers and all the others who were killed and injured that day," she said. "I wanted to be here to be a witness for my brother.
"I am a former attorney and I have attended a lot of trials, but this is really an extraordinary situation. Most defendants plead not guilty because they want to escape justice, but here are men who want to die as martyrs for Allah. They are proud of committing the worst war crime of modern times."
The attacks still cast a dark shadow across America, psychologically and politically. Indeed, President Barack Obama was accused by his critics of playing campaign politics last week when he released a video, narrated by his Democrat predecessor Bill Clinton, highlighting the mission a year ago that killed Osama bin Laden, the al-Qaeda chief.
But while bin Laden commanded the terrorist network, it was Mohammed who was its chief strategist.
Josh Meyer and Terry McDermott, co-authors of the recent book The Hunt for KSM, say that from for more than a decade, he devoted himself entirely to terrorism, admitting to involvement in more than 30 separate plots.
"KSM is a complex personality - charming, funny, smart, the kind of guy everyone liked immediately," they wrote. "But he was also a stone-cold killer who seemed to devise new plans for mass murder with every waking moment.
"There are still agents recruited by KSM on the loose, We might be dealing with KSM-designed or inspired attacks for years.
"More generally, KSM is the prototype of the modern, stateless enemy likely to haunt [the world] for decades to come."
After September 11, Mohammed hatched the plan for Richard Reid, the British "shoe bomber", to blow up a plane en route from Paris to Miami and co-ordinated the Bali nightclub bombings in 2002. He has also claimed responsibility in interviews for beheading Daniel Pearl, the American journalist captured and murdered in Karachi.
Khalid Sheikh Mohammed
Born: Kuwait, of Pakistani family. Used many aliases.
Captured: Rawalpindi, Pakistan, March 2003. Transferred to Guantanamo 2006.
Self-confessed "mastermind" of 9/11. Also told interrogators of involvement in 1993 World Trade Centre attack, 2002 Bali bombings and beheading of Daniel Pearl. Previously declared: "I am looking to be martyred."
Ali Abdul Aziz Ali (Ammar al-Baluchi)
Born: Pakistan. Nephew of Khalid Sheikh Mohammed.
Captured: Karachi, April 2003 (with bin Attash), reputedly with letter to bin Laden in pocket. Transferred to Guantanamo 2006.
Allegedly organised transfer of funds from Dubai to finance 9/11, taught hijackers how to behave and operate in US, helped arrange plane tickets, traveller's cheques, and hotel reservations.
Walid bin Attash
Member of prominent Saudi family close to bin Laden, lost leg fighting in Afghanistan.
Captured: Karachi, April 2003. Transferred to Guantanamo 2003.
Accused of selecting and training some of hijackers after working as bin Laden's bodyguard. Allegedly also involved in preparing 1998 US embassy bombings in East Africa and 2000 attack on USS Cole.
Mustafa al Hawsawi
Born: Saudi Arabia.
Captured: Pakistan, March 2003.
Transferred to Guantanamo, Sept 2003, moved overseas by CIA in 2004, returned to Guantanamo 2006.
Allegedly worked with bin Laden in Afghanistan.
Accused of organising and financing September 11 attacks, working with Aziz Ali; said to have shared credit card with Khalid Sheikh Mohammed.
Ramzi bin al-Shibh
Captured: Karachi, Pakistan, September 11 2002. Held overseas by CIA, transferred to Guantanamo 2006.
Accused of being "key facilitator" of 9/11 attacks. Lived with several of the hijackers in Hamburg, Germany; wired them money in US, passed information from al-Qaeda leaders. Tried to enrol in Florida flight school but refused visa; may have been missing "20th hijacker".
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He has previously chosen to brag about his role as the mastermind of the worst terrorist atrocity in history.
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