Khalid Sheikh Mohammed is accused of masterminding the attacks on the World Trade Center and other US targets
Scenes of chaos have gripped a US military court in Guantanamo Bay where five alleged 9/11 plotters were due to appear together before a judge.
All five, including alleged mastermind Khalid Sheikh Mohammed, initially refused to attend the hearing.
The judge rejected prosecution calls for them to be compelled to attend but three did appear after a recess.
The hearing focuses on whether two of the accused are mentally competent to represent themselves.
The 11 September 2001 attacks, in which hijacked airliners were flown into buildings in New York and Washington, killed nearly 3,000 people.
Microphone cut off
Walid Bin Attash, Ali Abd al-Aziz Ali and Mustafa Ahmad al-Hawsawi appeared in the courtroom after the recess.
Relatives of 9/11 victims were watching the hearing
But Mr Hawsawi asked to leave again after he was refused the opportunity to speak. He was escorted out by US military guards.
Mr Bin Attash asked the judge whether he could question a witness.
When told he could not, he asked: "Even if he told lies?"
When the judge again insisted he could not question the witness, Mr Bin Attash replied: "This is good justice!"
Thursday's hearing was meant to focus on whether Mr Hawsawi and a fourth defendant, Ramzi Binalshibh, were mentally competent to represent themselves.
The US military lawyer representing Mr Binalshibh said he suffered from a delusional disorder.
When the lawyer went into detail about how he had been deprived of sleep, her microphone was cut off.
The fifth defendant, Mr Sheikh Mohammed, refused to attend the hearing.
Described by US intelligence as "one of history's most infamous terrorists", he earlier admitted being responsible "from A to Z" for 9/11.
Believed to be the number three al-Qaeda leader before his capture in Pakistan in March 2003, he told a pre-trial hearing at Guantanamo in December 2008 that he wanted to plead guilty to all charges against him.
The Obama administration has yet to announce how the five defendants will be tried or where they will be held if the camp at Guantanamo Bay is closed by January 2010, as promised.
US President Barack Obama has ordered a halt to military commissions at Guantanamo Bay but preparations continue for military trials and prosecutors say they are ready to begin proceedings against 66 detainees.
Mr Obama has conceded that some will be tried in military courts once Congress passes legislation which puts in place a number of legal safeguards.