Moussaoui escaped a death sentence over the September 11, 2001 strikes because jurors did not believe US government arguments that he had played an integral role in the murder of nearly 3,000 people on that day.
Yet, in what some have hailed as a victory for traditional justice, he was brought to trial in the US criminal justice system and jailed for life, after what many legal observers have termed a fair trial.
However, the mastermind of the September 11 attacks, Khalid Sheikh Mohammed; Ramzi bin Al Shibh, the money-mover behind the strikes; and the supposed intended 20th hijacker, Mohammad Al Khatani, are in US hands.
So far, there is no sign that they will have their day in court.
And the head of Al Qaeda, Osama Bin Laden, remains at large, tormenting America over crackly tapes - despite US vows to bring him to justice over September 11.
Mohammed, also known as KSM, even played a key role in the Moussaoui trial - through a summary of his interrogations at a secret US jail, in which he directly contradicted Moussaoui's claims to have been a September 11 plotter.
"The fact of the matter is, we have real people who were actually involved, including the mastermind, the top guy, and we are not bringing them to justice," said David Cole, a law professor at Georgetown University in Washington.
Prosecutor David Novak stirred intrigue when he referred to Mohammed and other top Al Qaeda plotters in closing arguments at the Moussaoui trial.
"The KSMs of the world, the Bin Shibhs and all those other people ... they're going to have their day, they're going to face justice, just like this defendant does, when their interrogation time is over," Novak said.
But US authorities have given no sign they are considering any kind of criminal proceeding or even a military tribunal for suspects like Mohammed, who has simply disappeared since his arrest in 2003.
A decision on Mohammed's future will be made "after careful evaluation of the circumstances of this particular case," Attorney General Alberto Gonzales said in Vienna this week.
But even if President George W. Bush's administration did decide to reverse course and try Mohammed, held as "an enemy combatant" under new practices in the US "war on terror," there are huge hurdles.
Much of the testimony that would be brought into such a case may be inadmissible in a civilian court. Defense lawyers would argue that their client had no access to lawyers and that confessions may have come under duress.
The US government may also be concerned that such a trial could expose classified intelligence information, while defense lawyers would likely demand that such data be made public so they could build their case.
"I think the government has painted itself into a corner by adopting the short-sighted tactic of employing coercive interrogation," said Cole. "Once you have tortured someone, it is very hard to put that person on trial."
When Moussaoui left court on Wednesday, after the jury delivered its verdict - life in prison with no chance of parole, rather than execution - he shouted: "I won."
Judge Leonie Brinkema hit back the next day, saying he had certainly not emerged triumphant.
"In terms of winners and losers, it is pretty clear who won yesterday and who lost yesterday," the judge said, arguing that US justice had triumphed, and not the Al Qaeda conspirator, who will spend the rest of his life in prison.
Still, some observers believe the US government suffered a serious defeat after electing to seek the death penalty for Moussaoui but failing to secure it in a process that presented him with a megaphone for his propaganda.
While those whom defense lawyers called "the real terrorists" were either at large or in secret US prisons, the only prosecution over September 11 was left looking inadequate.
Prosecutors tried, and ultimately failed, to convince jurors that Moussaoui's lies while in jail in August 2001 allowed the September 11 conspirators time and space to carry out the attacks.
"Their theory was quite fragile," said Eugene Fidell, professor of law at American University. "It is not a surprise that the jury did not find what the government was selling. I think it was ill advised," he said. "Having tried to get the death penalty and not getting it is worse than not having tried ... in terms of public perceptions."
All the while, Bin Laden has evaded US capture and continued to berate and frustrate the US government - most recently with his latest threatening tape, released on April 23.
© 2006 Agence France-Presse
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