At the start of the trial of confessed al-Qaida member Zacarias Moussaoui in Alexandria, Virginia, on Monday, the district judge, Leonie Brinkema said that the jury should be guided by "reason and your sense of justice".
Moussaoui, 37, was arrested the month before the 11 September 2001 attacks which killed about 3000 people, after arousing suspicion at a flight school.
As he has pleaded guilty to the six charges against him, three of which carry the death penalty, the jury has been selected solely to decide the defendant's punishment.
Brinkema said: "In making this very difficult decision about punishment, you must be guided by reason and your sense of justice and not by bias, prejudice or sympathy for or against the defendant or the victims."
Dressed in a green prisoner jumpsuit, Moussaoui sat passively through the hearing, rubbing his beard as 17 jurors were finally selected from an initial group of 500 people.
There was no repeat of his outbursts during earlier jury selection hearings, when he berated the judge, his court appointed lawyers - who he has repeatedly tried to "sack" - and declared "I am al-Qaida", earning himself repeated expulsions.
In a statement in April 2005, Moussaoui admitted to six counts of conspiring with al-Qaida to fly airliners into US government buildings.
There was no repeat of previous
outbursts from Moussaoui
But he maintains he was to fly a plane into the White House as part of a follow-up wave of attacks rather than participate in the September 11 strikes.
According to the independent US commission which investigated the attacks, Moussaoui, who has proclaimed himself a "slave of Allah", travelled to Malaysia and met Khaled Shaikh Mohammed, who is credited with being the brains behind the 2001 attacks.
He then travelled to the US and undertook flight training in Oklahoma and Minnesota.
Life or death
Prosecution lawyers were due to begin the case with a 45-minute opening statement laying out the case for Moussaoui's execution.
The jury will first be asked to decide whether Moussaoui is eligible for the death penalty, and whether he should receive it.
The alternative punishment is life in prison, without possibility of parole.
Brinkema said evidence from six "enemy combatants" may be presented during the trial which was delayed for years due to appeals over Moussaoui's access to al-Qaida detainees he said could help his case.
Some observers have criticised the US government for seeking the death penalty which they said presented Moussaoui with a public forum and the chance to turn himself into a martyr for his cause.