Prosecutors in the sentencing trial of Zacarias Moussaoui's say he conformed to al-Qaeda training by not revealing details of the 11 September plot.
Moussaoui has said "I am al-Qaeda"
On the second day of Moussaoui's trial, government lawyers sought to prove that he deliberately kept silent and is responsible for nearly 3,000 deaths.
Moussaoui's defence argued that the FBI knew more of the attacks than he did.
Moussaoui, arrested shortly before the 9/11 attacks, is the only person directly charged over them.
The self-confessed member of al-Qaeda pleaded guilty in April to six charges of conspiracy.
At the time, he said he was not meant to be part of the 9/11 attacks, but was part of a broader conspiracy to use airplanes to strike the White House.
Judge Leonie Brinkema told the jury on Monday that the prosecution must prove that Moussaoui's actions directly contributed to deaths on 11 September.
If the jury decides they did, it will be asked to consider the death penalty.
'Trained to lie'
In the courtroom on Tuesday, Moussaoui's mother, Aicha el Wafi, sat three rows behind her son but he ignored her except for a glance, the Associated Press news agency reported.
Al-Qaeda expert and FBI special agent Michael Anticev took to the witness stand for a second day on Tuesday, where prosecutors asked him to read from one of the group's training manuals.
One passage said operatives were trained to lie if they were captured, in order to shield co-conspirators and allow planned attacks to go ahead.
Asked about warning signals before 9/11, Mr Anticev said he didn't "think anybody was looking at using aircraft as weapons", but later said that the FBI had been aware of al-Qaeda plans to fly airliners into the Eiffel Tower in France.
Another FBI agent to testify, James Fitzgerald, said Moussaoui had links to some of the people that had been in touch with the 19 hijackers responsible for the attacks on New York and Washington.
The defendant, a 37-year-old Frenchman of Moroccan origin, was arrested in Minnesota a month before the attacks after arousing suspicion at a flying school. He initially told federal agents he was training as a pilot only for personal enjoyment.
If he is spared the death sentence, Moussaoui will spend the rest of his life in prison. The trial, being held in Virginia, could last for as long as three months.