Zacarias Moussaoui is so far the only person to be charged in the United States in connection with the 11 September attacks. BBC News Online answers key questions about his trial.
What is Moussaoui charged with?
Zacarias Moussaoui, a 35-year-old French citizen of Moroccan origin, is the only person so far charged in connection with the 11 September attacks on America.
The US Government regards Mr Moussaoui as the "20th hijacker" - and accuses him of conspiring with the 19 other men alleged to have carried out the attacks on New York and Washington.
Mr Moussaoui was arrested on immigration charges three weeks before the suicide hijackings after he aroused suspicions at a flight school in Minnesota.
He is charged on four counts that carry the death penalty in the case of conviction - conspiracy to commit acts of terrorism, to commit aircraft piracy, to destroy aircraft and to use weapons of mass destruction.
He is also charged with conspiracy to kill US Government officials and destroy US Government property - two lesser charges.
How strong is the evidence against Moussaoui?
Prosecutors have said that Mr Moussaoui's actions mirror those of the 19 alleged hijackers behind the 11 September attacks.
He went through flight instruction, is alleged to have received money from a terrorist suspect and is alleged to have been trained at an al-Qaeda camp.
The US authorities have not offered evidence of a direct link between him and the hijackings.
Some legal experts have said that the evidence against Mr Moussaoui is circumstantial and that there is only evidence of association with some of those who are alleged to have carried out the attacks.
He has said in court that he provided a guest house for al-Qaeda members, but insisted that he was not involved in the attacks.
Why did Moussaoui plead guilty and then withdraw his plea?
Mr Moussaoui pleaded guilty to some of the charges against him in July 2002, but then changed his mind and withdrew the plea.
The hearing judge rejected that plea, ordering him to take a week to consider his decision.
At one court appearance, Mr Moussaoui said he was a member of al-Qaeda and had sworn a pledge of allegiance to Osama Bin Laden.
It is not exactly clear why Mr Moussaoui entered and then reversed his plea. One suggestion is that he did not understand the gravity of his situation and the charges against him.
Another theory is that he wanted to short-circuit a long jury trial that he felt would inflame public opinion against him.
Is Moussaoui fit to stand trial?
There have been questions about Mr Moussaoui's state of mind, and his fitness to stand trial.
His behaviour in court has been erratic and at times he has not appeared to understand what is happening to him - specifically the seriousness of the charges against him.
He has insisted on representing himself, accusing his court-appointed lawyers of trying to kill him.
As the only person charged by the US Government in connection with the 11 September attacks, he is under tremendous pressure that is only increased by this isolation.
The judge in the case has ruled that he is mentally fit to stand trail.
Could the case end up before a military tribunal?
Yes, because of a dispute over whether an alleged al-Qaeda leader, Ramzi Binalshibh, should be allowed to testify before the court.
Prosecutors say Mr Moussaoui received funds from Mr Binalshibh, who allegedly helped finance the 11 September attacks.
Mr Moussaoui's lawyers have always disputed this. When Mr Binalshibh was arrested in Pakistan in September 2002, they requested permission to interview him, saying his deposition was crucial in proving their clients' innocence.
The Virginia judge handling the case, Leonie Brinkema, has agreed in principle to an interview over closed-circuit television.
The decision has infuriated prosecutors. They object to any such move on security grounds, and have filed an appeal.
The US Justice Department has so far been keen to try Mr Moussaoui in an open court, where the unanimous verdict required for a conviction would carry weight.
But the administration could change its mind it views a public trial as a threat to national security.
If it forced to allow Mr Binalshibh to testify, the option of a closed military tribunal - where only a two-thirds majority is required for a verdict - remains available to the US Government.