Jury selection has begun at the start of the trial to decide whether confessed al-Qaeda operative Zacarias Moussaoui should be executed for his role in the 11 September attacks.
Moussaoui is the only man convicted in connection with the attacks in the US in 2001.
The 37-year-old has admitted to conspiring with al-Qaeda to carry out the attacks in which almost 3,000 people were killed.
It is thought the US jury selection process will take around one month to complete.
Jurors will be asked to rule on whether Moussaoui is eligible to be put to death for his crimes and then must decide if he should get the penalty.
Moussaoui, born in France to Moroccan parents, was arrested by the FBI in August 2001.
At that time he was attending a flight school in Minnesota.
Federal prosecutors allege that by not co-operating then Moussaoui prolonged the conspiracy of the 9/11 attackers.
After a long and complex legal process, Moussaoui pleaded guilty in a preliminary hearing in April last year to conspiring with the 11 September attackers but denied he was directly involved with planning those attacks.
His lawyers contend that he suffers from a mental illness and his conversion to radical Islam in London in the 1990s was linked to this state of mind.
