Zacarias Moussaoui may be put to death for his role in the Sept. 11, 2001, attacks that killed nearly 3,000 people, a jury found Monday. The jury will consider in a second penalty phase that begins Thursday whether to recommend death or life imprisonment for the Al Qaeda devotee.

The sentencing phase for Moussaoui, who pleaded guilty last year to six terror-related conspiracy charges, will involve the testimony of victims' family members and is expected to be protracted and highly emotional. If the jury is not able to reach a unanimous conclusion, U.S. District Judge Leonie Brinkema must impose the lesser sentence of life imprisonment without the possibility for parole.

The Justice Department said it was "pleased" with the verdict. Moussaoui is the only person to be charged and convicted in relation to the worst act of terrorism ever on U.S. soil.

• Click here for Moussaoui's case history (FindLaw).

The jury had to find that Moussaoui was responsible for at least one death in the attacks in order for him to be death-penalty eligible. The case is a precedent-setting one, in that Moussaoui was not convicted of having a direct role in the Sept. 11 attacks or plot.

"I thought I would be elated but I wasn't," said a visibly emotional Abraham Scott, whose wife, Janice Marie Scott, was killed in the attacks.

Scott said he was absolutely certain the jury reached an appropriate decision.

"To describe him, as well as those who perpetrate acts of terrorism, I describe him like a dog with rabies," Scott told reporters outside the Alexandria, Va., courtroom. "The only cure is to put him or her to their death."

Moussaoui caught the eye of an FBI agent in August 2001 after the Frenchman paid a Minnesota flight school cash to teach him how to fly a commercial airliner. An FBI agent's repeated requests to supervisors to search Moussaoui's belongings were denied; Moussaoui was arrested and detained on immigration charges.

At the time, Moussaoui denied any ties to terrorism. The jury on Monday unanimously found that he lied knowing that a person would die and that at least one 9/11 victim died as a direct result of Moussaoui's lies.

"I find the decision of the jury to be very problematic," said Jonathan Turley of George Washington University Law School. "I do not see in the evidence any basis to conclude beyond a reasonable doubt that had Moussaoui revealed what he knew, we would have prevented 9/11."

If Moussaoui is executed for lying about a crime he did not commit, a dangerous precedent will be set, Turley said. In that event, a Supreme Court challenge could be on the horizon.

"He's basically being put to death for the act of omission. There would be no limit to how far this type of theory could be used," Turley said.

But in many respects, Moussaoui has dug his own grave, Turley conceded. Unlike most defendants in capital murder proceedings, his anti-Semitic and anti-America diatribes seemed to have been intended to repulse the jury.

"There comes a point where the jury simply is not willing to save a defendant from his own incriminating statements," Turley said. "But just because Moussaoui wants to be put to death as a 9/11 co-conspirator, does not mean we are supposed to yield to that demand."

Moussaoui pleaded guilty in April 2005 to conspiring with Al Qaeda to commit acts of terrorism, using weapons of mass destruction and other terror-related crimes. Originally considered the "20th hijacker" when he was charged in December 2001, the intelligence community has since come to believe Moussaoui had no direct role in the attacks.

High-ranking members of Al Qaeda have told investigators that Moussaoui was considered too unstable to be included in the plot, and that he was earmarked for a second wave of attacks. After he pleaded guilty to the six terrorism-related counts last April, Moussaoui quickly about-faced and insisted he had no direct role in the attacks.

"I will fight with every inch against the death penalty," he vowed.

But in recent months, the 37-year-old has changed his story, apparently in an effort to become a martyr.

Last month, Moussaoui offered to testify for prosecutors against himself. And last week, Moussaoui appeared to drop a bombshell in the case when he testified that he knew about the attacks ahead of time and that he and failed shoe-bomber Richard Reid were slated to hijack a fifth airliner and crash it into the White House.

Moussaoui has never shied from his allegiance to Al Qaeda, and his erratic courtroom behavior and outbursts have raised questions about his mental competency. At several points throughout his four-year-long trial, Brinkema also seemed to question his competence.

Moussaoui has still not squelched his tirades despite having been barred from the courtroom for past outbursts. He appeared to be praying as the jury's verdict was read, but once Brinkema and the jury left the courtroom Moussaoui began to yell "Allah Akbar" and "God curse you all, you will never get my blood," according to a FOX News producer who was in the courtroom.

The jury, compromised of nine men and three women, deliberated for more than 12 hours after receiving the case last Wednesday. The jury asked Brinkema one question about the definition of "weapon of mass destruction," and were told that a plane used as a missile qualified as a WMD.

A Frenchman of Moroccan descent, Moussaoui was in jail at the time of the attacks. Prosecutors argued federal agents would have been able to thwart or at least minimize the attacks if Moussaoui had revealed his Al Qaeda membership and his terror plans when he was arrested and interrogated by federal agents.

The legal community has been watching Moussaoui's case closely, not just for its historical significance but for the precedent it would set for capital murder convictions.

Throughout the trial, Moussaoui's defense argued that the well-documented missteps of intelligence agencies leading up to the attacks cast doubt on the government's assertion it could have prevented the attacks.