Prosecutors argued his silence kept the plot under wraps and allowed it to succeed; defense lawyers said a dysfunctional federal law enforcement system never would have stopped the hijackers anyway.
Both sides agreed that Moussaoui was a liar and a manipulator -- but disagreed on what points they thought should be disbelieved.
Prosecutors said he lied to FBI agents when he was arrested 25 days before terrorists hijacked four passenger jets and crashed them into the World Trade Center, the Pentagon and a Pennsylvania field. Defense lawyers said he was lying now to inflate his importance in the plot, "trying to write a role in history for himself, when the truth is he's just an Al Qaeda hanger-on."
Prosecutors said they believed Moussaoui when he testified that he was in the United States to pilot a fifth plane into the White House on the morning of Sept 11. At the same time, they branded him a liar who tried to deceive FBI agents upon his arrest.
Defense lawyers also took an untraditional tack in their closing arguments. Faced with a client they could not control, they repeatedly castigated him as untrustworthy, someone who was trying to manipulate the jury. And they all but accused Moussaoui, who in the past had claimed he was to be part of a second wave of attacks, of committing perjury when he testified about being the fifth Sept. 11 pilot.
The 37-year-old Frenchman was apprehended on visa violations while taking jet simulator lessons in Minnesota.
Seventeen potential jurors heard the evidence, and on Wednesday Judge Leonie M. Brinkema randomly chose 12 of them -- nine men and three women -- as the jury. The five others were named alternates.
Wednesday's session concluded the first phase of the sentencing trial, convened because Moussaoui pleaded guilty to two capital murders last April.
The jury must decide several key questions:
* Did Moussaoui lie to FBI agents in Minnesota on Aug. 16 and 17, 2001, when he claimed he was a tourist?
* If so, did he lie "contemplating" that someone would die as a result?
* Were his lies responsible for any of the 2,792 deaths on Sept. 11?
A decision that he lied -- and that people died because of his lies -- must be unanimous.
If all 12 jurors agree with the prosecution that Moussaoui is eligible for the death penalty, the sentencing trial will move into a second phase. There, the two sides will present evidence trying to persuade the jury to give him one of two options: the death penalty or life in prison with no parole.
If even one juror does not agree that he is eligible for the death penalty, the case will end and Moussaoui will be sentenced to life without parole.
The jury deliberated about 90 minutes Wednesday afternoon and is to return today.
Prosecutors tried to persuade jurors that they should set aside any uncertainty about whether Moussaoui was to join the Sept. 11 hijackers or commandeer a plane at a later date. They urged the jury to concentrate on what they said was their proof that Moussaoui's lies to FBI agents in Minnesota cost the government the chance to locate the Sept. 11 plotters and stop the hijackings.
"It doesn't matter whether he was part of the first wave, part of the second wave or the 20th wave," Assistant U.S. Atty. David Novak told the jury. "What matters is the knowledge he had. The key is the information that he had."
But Edward B. MacMahon Jr., one of Moussaoui's court-appointed lawyers, argued in his closing statement that Moussaoui was never groomed for a Sept. 11 role and that Al Qaeda leaders had dismissed him as an annoyance -- not a terrorist who could be trusted.
MacMahon said the FBI and CIA were not cooperating with each other enough in the summer of 2001 to find the hijackers, even with Moussaoui's help.
"That's the truth," he said. "The sad truth, but it's the truth."