Zacarias Moussaoui, the only individual criminally charged by the US with participating in the September 11 plot, pleaded guilty on Friday to charges that could carry the death penalty.
Mr Moussaoui appeared in a Virginia court against the wishes of his lawyers and said he was guilty of all six counts brought against him by US prosecutors.
One of his defence lawyers, Alan Yamamoto, told the court that Mr Moussaoui “is facing the possibility of death or life in prison. He told me he understands that,” according to an AP account of the hearing.
The plea marks the most significant victory for US prosecutors in the investigation of the al-Qaeda attacks in New York and Washington, which killed nearly 3,000 people. Alberto Gonzales, US attorney-general, said: “The fact that Moussaoui participated in this terrorist conspiracy is no longer in doubt. In a chilling admission of guilt, Moussaoui confessed to his participation.”
The US indicted him on a range of charges including conspiracy to commit terrorism, to hijack and destroy aircraft, and to murder government employees.
Mr Gonzales said Mr Moussaoui had been “personally selected” by Osama bin Laden, al-Qaeda leader, to carry out the attacks.
His defence attorneys on Friday sought to prevent him from entering the plea, filing a motion with the court to have him declared unfit. But Judge Leonie Brinkema told the court she had discussed the plea extensively with Mr Moussaoui and was certain he knew what he was doing.
Mr Moussaoui, a French national, was arrested in August 2001, when officials with the Federal Bureau of Investigation learnt that he had paid $6,800 in cash to a Minnesota flight school to learn to fly large aircraft. Warnings from local officials to national FBI headquarters were largely ignored, and the FBI did not succeed in unravelling the plot before the attacks were carried out.
A trial must still be held to determine his punishment, and the US made clear on Friday it would seek a death sentence.
Mr Moussaoui had tried once before to enter a guilty plea, but was barred by the court. At other times, however, he had declared his innocence, claiming that he had not participated directly in the September 11 plot.