Zacarias Moussaoui - the only person to be charged in the United States in connection with the 11 September attacks - has been spared the death penalty and faces life imprisonment. The BBC News website answers key questions about his case.
What charges did Moussaoui admit?
Moussaoui, a 37-year-old French citizen of Moroccan origin, pleaded guilty in April 2005 to conspiring with the 19 men who carried out the suicide hijacking attacks on New York and Washington.
He admitted six charges of conspiracy:
- To commit acts of terrorism
- To use weapons of mass destruction
- To destroy aircraft
- To commit aircraft piracy
- To murder US employees
- To destroy property
The first four charges carry the death penalty.
Moussaoui has sometimes been referred to as the "20th hijacker", after suggestions that he would have joined the other 19 men had he not been arrested three weeks earlier on immigration charges after behaving suspiciously at a flight school.
During the first phase of the trial, he said he was to have flown a plane into the White House on 11 September.
What did the first phase of the trial decide?
The jury in Alexandria, Virginia, ruled that Moussaoui was eligible to face the death penalty.
The nine men and three women decided unanimously that Moussaoui's lies to US investigators after he was arrested led directly to at least one death on 9/11.
Moussaoui had not entered into any plea agreement with prosecutors and said in court that he expected "no leniency".
Previously he had said he would fight the death penalty.
What happened in the second phase?
While the first part determined that Moussaoui's crimes were worthy of the death penalty, the second part will consider whether Moussaoui himself should be executed for them.
Moussaoui's lawyers argued that he did not deserve to be executed because of his difficult upbringing and possible mental illness. They also said execution would only serve to fulfil his dream of martyrdom.
In the first phase, they argued that American security agencies were so bureaucratic they could not have acted to stop the 9/11 attacks even if Moussaoui had co-operated with them.
The second phase was heard by the same jury that heard the first part.
Was Moussaoui fit to be tried?
There were questions about Moussaoui's state of mind and his fitness to stand trial.
His behaviour in court was erratic and at times he did not appear to understand what was happening to him or the gravity of the charges against him.
He pleaded guilty to some of the charges against him in July 2002 but then changed his mind and withdrew the plea.
He accused his first court-appointed lawyers of trying to kill him and fired them but in 2003 Judge Leonie Brinkema revoked his right to defend himself after Moussaoui sent a string of angry, invective-filled letters to court officials.
After a closed meeting with Moussaoui in mid-April 2005, Judge Brinkema ruled he was mentally fit to enter a plea.
During the first phase of his sentencing trial, he offered to take the stand against himself, reportedly saying it was better to die in battle than in jail.
Why did the case drag on for so long?
Moussaoui's case was long tied up in appeals over whether he could have access to top al-Qaeda suspects to aid his defence.
These included Ramzi Binalshibh, who allegedly helped finance the 11 September attacks.
Moussaoui's request to interview detainees pitted his right to a fair trial against the US government's concerns over national security.
The dispute over witness access could also come up again at his trial.
What evidence was there against Moussaoui?
Prosecutors have said that Moussaoui's actions mirror those of the 19 hijackers behind the 11 September attacks.
He went through flight instruction, received at least $14,000 from a terror suspect and trained at an al-Qaeda camp in Afghanistan.
Moussaoui said in court he was being trained on a Boeing 747 simulator and intended to use such a plane to strike the White House.
At an earlier hearing he said he had provided a guest house for al-Qaeda members.