Prosecutors argued that even though he did not take part in the attacks, Moussaoui deserved the death penalty because he was aware of the terrorists' plan to crash planes into the World Trade Center and Pentagon but did not tell investigators, thus ensuring the deaths of over 3,000 people.
On April 3, a federal jury in Alexandria, Virginia, agreed. "By this verdict, the jury has found that death is a possible sentence in this case," Edward Adams, a U.S. district court public information officer, informed reporters.
The jury ruled Moussaoui eligible for the death penalty because his actions led directly to at least one death on September 11.
Moussaoui reacted to the verdict announcement by cursing the court, shouting, "You'll never get my blood, God curse you all."
Earlier in his trial, Moussaoui claimed that if he had not been jailed, he would have been flying a fifth airplane and planned to crash it into the White House -- on the orders of Al-Qaeda mastermind Osama bin Laden.
The trial now moves to its sentencing phase, during which the jury of nine men and three women will decide whether Moussaoui should in fact be put to death. The only uncomfortable question that hangs over the proceedings is whether Moussaoui is a credible witness.
Chris Bellamy, of Britain's Cranfield University, says Moussaoui's court testimony, in which he linked himself to bin Laden, was significant. But doubts about the defendant's truthfulness remain.
"This is a breakthrough because bin Laden's presence and whereabouts have always been extremely shadowy," Bellamy says. "So for somebody actually to say in court, 'Yes, I had direct orders from him,' is unusual. Whether that is true or not, I'm not aware."
That is a point taken up by Magnus Ranstorp, a terrorism expert at Sweden's National Defense College, who questions what prompted Moussaoui to make his admission.
"I'm not sure at what sort of face value you can take an admission like this, unless there has been a deal," he says. "You can see that there may have been a deal done in terms of reducing the possibility of a death sentence. Why otherwise would someone admit to this?"
A desire to be a martyr or mental illness are two other possibilities being floated by commentators.
Successfully Trying Terrorists
Regardless of the motives, Bellamy says Moussaoui's court performance proves that open trials can be effective in terrorism cases, giving even more justification to those who say the U.S. detention facility at Guantanamo Bay, Cuba, should be closed and its detainees judged by civil courts.
"This case, I think, shows that to try somebody in an open court may give you precisely the result that you want, which is that the person effectively admits to complicity and to taking part in a plot and to getting orders from the big boss," Bellamy says. "I firmly believe myself that secret trials and detention without trial are quite unacceptable in a democratic society with the rule of law and that if we do continue to conduct ourselves in that way, we are essentially doing what the terrorists want. Because one of the classic aims of terrorism is and has always been to provoke repression."
Moussaoui, for his part, said he believed his fate was "in God's hands."