Zacarias Moussaoui, a 37-year-old Frenchman of Moroccan descent, was arrested in August 2001, weeks before the world's deadliest terror attack, which killed nearly 3000 people.
But prosecutors contend he knew in advance of the strike on New York and Washington by suicide pilots with hijacked airliners, and is culpable as he could have prevented them by spilling the plans to FBI agents.
The 12-strong jury plus six alternates will be finalised on Monday morning after a three-week selection process. Opening statements are due to begin that same afternoon.
The jury will first be asked to decide whether Moussaoui is eligible for the death penalty, and if he is, whether he will receive it. The alternative punishment is life in prison, without possibility of parole.
In a statement in April 2005, Moussaoui admitted to six counts of conspiring with al-Qaida to fly airliners into US government buildings.
But he has maintained he was due to fly a jetliner into the White House as part of a follow-up wave of attacks and not the September 11 strikes.
'I am al-Qaida'
The death penalty phase of the trial, expected to last between one and three months, promises to lace the Moussaoui saga with new drama.
On the first day of jury selection last month, Moussaoui sparked uproar, berating the judge and his court appointed defence lawyers, and earning repeated expulsions from the heavily guarded courtroom.
Moussaoui was arrested in
"I am al-Qaida," he declared, prompting the judge to tell him he was his own worst enemy.
But after he was surprisingly let back into proceedings by Judge Leonie Brinkema - who had threatened to make him watch his trial on television in his cell - he sat mute through subsequent jury selection hearings.
Confining himself to sneers at the press benches and public galleries each time proceedings recessed, Moussaoui often muttered "God curse the United States" on his way back to the cells.
Moussaoui, a self-proclaimed "slave of Allah" will be on trial in Alexandria, Virginia, a suburb of Washington DC, just a few kilometres from the Pentagon, charred in the 2001 attacks.
His road to the Alexandria courthouse started when he had a rough childhood in southwestern France, before hearing the call to jihad for the first time at mosques in London, where he studied international business.
According to the independent US commission which investigated the September 11 attacks, Moussaoui travelled to Malaysia and met Khalid Shaikh Muhammad, who is credited with being the brains behind the 2001 attacks.
He then travelled to the United States and took flying lessons in the state of Oklahoma before moving again to Minnesota where he trained on a 747 Flight simulator.
Prosecutors have never been able to prove that he would have taken part in the September 11 attacks had he not been arrested for overstaying his visa and arousing suspicion.
Some observers have speculated that he was a "spare" pilot, in case one of the other al-Qaida hijackers was taken out of circulation or had second thoughts.
The prosecution has signalled it plans to call relatives of victims of the September 11 attacks in the second phase of the trial if Moussaoui is judged liable to face the death penalty.
But defence lawyers may also embarrass the government, possibly by arguing that the FBI knew far more about the September 11 strikes than it has previously admitted.
One of Moussaoui's lawyers, Gerald Zerkin, has admitted that he faces "one of the hardest cases" in his attempt to spare his client from the death chamber.
But he cautioned that the government also faces a tough task: it must prove Moussaoui "did something that directly resulted in the death of victims from the 9/11 attacks".
And the wild card in the trial is Moussaoui's apparent determination to testify in his own defence.
"There will be surprises, but count on surprises from both sides," said Michael Mello, professor at Vermont Law school, who has closely followed the case.
Some legal observers have criticised the US government for seeking the death penalty for Moussaoui, and thus presenting him with a public forum, and the possibility of martyrdom.
Relatives of September 11 victims will watch the trial in five remote locations around the United States, by closed circuit television.