Zacarias Moussaoui in August 2001. (AP photo)

Zacarias Moussaoui, the so-called "20th hijacker," pleaded guilty Friday to all charges against him in connection with the Sept. 11 attacks on the United States.

He denied, however, having been part of the Sept. 11 scheme to attack the World Trade Centre in New York and the Pentagon in Washington.

Instead, he said, he was part of separate plot to crash an airliner into the White House at, possibly, another time.

Moussaoui's afternoon plea in an Alexandria, Va. courtroom capped a day of drama and confusion. Only hours earlier, his lawyers filed a motion in court arguing their client was incompetent.

But Moussaoui said in court he understood he could be put to death for his role in planning attacks on the U.S.

U.S. District Judge Leonie M. Brinkema accepted the plea, making the French citizen of Moroccan descent the lone person convicted in a U.S. court for the atrocity that killed nearly 3,000 people in 2001.

Moussaoui is charged with conspiracy to commit acts of terrorism, commit aircraft piracy, destroy aircraft, murder government employees and destroy property. He could face the death penalty in four of the six charges.

"The court is accepting today the defendant's six pleas of guilty to the six counts of the indictment," Brinkema said. "You are found guilty at this time," she told Moussaoui.

She said she had discussed Moussaoui's pleas at length with him earlier. "He has a better understanding of the legal system than some lawyers I have seen in court," the judge said.

Lawyers at odds with defendant

Defence lawyer Alan Yamamoto, the only lawyer Moussaoui would talk to in recent weeks said in court Moussaoui had told him he understood what he was doing.

But he also added that he was of a different opinion than Moussaoui. "When I have spoken to him, we have disagreed," Yamamoto said.

Moussaoui, also the only person charged in the U.S. for alleged involvement in the Sept. 11 attacks, had been expected to plead guilty to at least some of the six charges, four of which carry the death penalty.

But his lawyers filed papers with the court Friday that argued their client was not competent to plead guilty to crimes that carried a possible death sentence.

The motion came just hours ahead of a hearing before U.S. District Judge Leonie Brinkema, who earlier in the week met with Moussaoui and determined he was competent to enter such a plea.

Showcase trial averted

Moussaoui was charged three months after the attacks.

Moussaoui's trial was to have been an evidentiary showcase detailing the horror of al-Qaeda. Those plans have apparently been scuttled by Friday's guilty plea.

Jumping past a trial, Moussaoui lawyers would eventually be defending him in a sentencing hearing.

Role in plot unclear

Picked up in August 2001 after arousing suspicion at a Minnesota flight school, Moussaoui was transformed from an immigration violator into a terrorist defendant three months after the Sept. 11, 2001, attacks. Whether Moussaoui was intended to be a participant in the Sept. 11 attacks is unclear.

Khalid Shaikh Mohammed, the purported Sept. 11 mastermind, considered replacing the pilot of the plane that crashed in Pennsylvania with Moussaoui, according to the report by the Sept. 11 commission.

Mohammed, however, has told his interrogators that Moussaoui actually was being considered for a second wave of attacks still in the early planning stages.

Moussaoui seeking martyrdom?

Some legal experts say Moussaoui's decisions seem to make no sense, unless he wants to die.

One possibility is that "he was deprived of his martyrdom and feels the only way he can achieve that lofty state is simply to admit to the crimes," Washington defence attorney Richard Hibey told the Associated Press.

Doubts have been raised about his mental fitness. Moussaoui has been in solitary confinement for three years and has often acted against what appear to be his own best interests.

Moussaoui has tried to fire his lawyers, launched multimillion-dollar civil suits alleging prison abuse, and publishes on the internet his thoughts on the case, including attacks on Brinkema, his own lawyers and the U.S. government.