Before his testimony on March 27, the French national of Moroccan descent had already pleaded guilty to multiple counts of conspiracy, securing him a lifetime in prison.
At issue in this trial was whether Moussaoui had advance knowledge of the September 11 attacks when he was arrested by the FBI in August 2001.
At the time of his arrest -- and up until his testimony -- Moussaoui had insisted he was not part of the plot, which killed some 3,000 people in New York and Washington.
And prosecutors were having a difficult time proving a link. Indeed, the whole trial proceedings came into question at one point, according to Magnus Ranstorp, a terrorism expert at Sweden's National Defense College.
"There had been some government witnesses that had been apparently coached or at least alleged to have been coached," Ranstorp said. "And the judge [took] a severe view of this and [of] sanctioning this in terms of potentially excluding some of the critical evidence that was to be presented."
But in a gripping reversal on March 27, Moussaoui, speaking calmly and clearly, admitted lying to the FBI. He said that not only was he aware of the plot, he participated in it.
Moussaoui claimed that if he had not been arrested, he would have flown a fifth suicide mission aimed at the White House. He said he had discussed his "dream" of destroying the building with none other than Osama bin Laden himself, who gave him his blessing and his personal order to complete the mission.
Finally Link To Bin Laden
Moussaoui's testimony could now earn him the death penalty. It represents a major victory for U.S. prosecutors -- both because it gives them a successful prosecution for the September 11 attacks and because it directly links bin Laden to the operation.
The only uncomfortable question that hangs over the proceedings is whether Moussaoui is a credible witness.
"This is a breakthrough because bin Laden's presence and whereabouts have always been extremely shadowy," said Chris Bellamy of Britain's Cranfield University. "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 Ranstorp, who questioned what prompted Moussaoui to make his sudden 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 said. "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.
Benefits Of Open Trial
Regardless of the motives, Bellamy said 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 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," he said.
"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," Bellamy added. "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."