Moussaoui, a 37-year-old Frenchman, now faces a second phase of his sentencing trial to determine if he actually will be put to death for his role in the worst terrorist assault on the United States.
After less than three days of deliberations, the jury decided that Moussaoui had deliberately concealed from investigators in August 2001 that Osama bin Laden's terrorist network was planning the strikes on New York and Washington.
Prosecutors had charged during the trial that Moussaoui "lied with lethal intent".
The jury voted ‘yes’ to a key question in the trial whether at least one victim died on September 11 as a result of Moussaoui’s actions.
Moussaoui sat in his chair and prayed silently as the verdict was read out. He refused to stand.
"You'll never get my blood, God curse you all," he said
Sentencing hearing
The nine men and three women of the jury will now hear testimony on whether the Frenchman, who was in jail at the time of the attack, deserves to be executed for his role.
The testimony will include families of September 11 victims who will describe the human impact of the al-Qaeda mission that flew four jetliners into the World Trade Centre, the Pentagon and a Pennsylvania field.
Court-appointed defence lawyers, whom Moussaoui has tried to reject, will summon experts to suggest he is a schizophrenic after an impoverished childhood during which he faced racism in France over his Moroccan ancestry.
Trial arguments
During the trial, opposing lawyers in the case sketched out sharply different pictures of Moussaoui, the only man tried in the United States in connection with the attacks.
In final arguments last week, prosecutor David Raskin said: "Zacarias Moussaoui came to this country to kill as many Americans as he could. In this trial you have heard from the defendant himself that is exactly what he did."
However defence counsel Edward MacMahon hit back, saying: "Moussaoui was not involved in the 9/11 plot, no matter what he says."
Families divided
The decision that Moussaoui is eligible for the death penalty has drawn a mixed reaction from family members affected by the September the 11th attacks.
"This must be looked in a larger context of the war on terrorism, if he gets the death penalty, then he becomes a martyr, a symbol to rally more of the radical Muslims," said Charles Wolf, whose wife was killed in the attack on New York's World Trade Centre.
"Are we playing right into their hands if we give him the death penalty? Do we make ourselves weaker? Are we going to let ourselves be manipulated into killing him? Because why else would he have come out and said 'I did this, this and that'?"
"I don't want to play into his hands, I want to be smarter than that," Mr Wolf told AFP.
Lorie van Auken, a New Jersey widow, disagreed with the jury's decision, "I don't think he contributed to what happened on September 11th. I think he's been scapegoated," she said.
Ms Van Auken questioned the reasoning behind the jury's decision, which accepted the prosecution's argument the Moussaoui was responsible for the attacks because he withheld information that could have prevented them.
"I didn't know that to withhold information made you eligible for the death penalty ... the CIA withheld information from the FBI," she said.
But outside of the Alexandria courtroom, some relatives cheered the jury’s decision, "The jury did a long, good job," said Rosemary Dillard, whose husband Eddy was killed in the attacks.
Abraham Scott, whose wife Janice died when a hijacked plane was flown into the Pentagon in Washington, struggled to express his feelings.
"It's very emotional ... it's hard to describe. I thought I would be delighted but I wasn't. I still feel sorry for him," he said.
"I don't think Moussaoui is totally to blame, even though I think he deserves death. I equally blame the government," he said in a choked voice.