Zacarias Moussaoui (search) pleaded guilty on Friday to participating in the Sept. 11, 2001, attacks that killed nearly 3,000 people in New York, Washington and Pennsylvania.
"Zacarias Moussaoui now joins shoe bomber Richard Reid (search), John Walker Lindh (search) and more than 200 other individuals who have pled [sic] guilty or been convicted of terror-related charges since 9/11," Attorney General Alberto Gonzales said in a press conference.
Four of the six counts against the Moroccan-born French national carry a maximum sentence of death.
"We are seeking the death penalty in this case," Gonzales said.
When asked by the judge if he understood the implications of his plea, Moussaoui answered: "I don't expect any leniency from the Americans."
He calmly answered U.S. District Judge Leonie Brinkema's (search) questions. Brinkema, in turn, told the court that Moussaoui had a better understanding of the law than some lawyers.
Moussaoui's plea capped more than three years' worth of legal battles: Moussaoui versus his attorneys, Moussaoui versus the judge, and the judge versus the Justice Department.
Moussaoui, who has been kept in solitary confinement in all that time, has since grown a full beard, though his head is still shaved.
He appeared subdued and almost respectful on Friday as he stood before Brinkema, whom on several occasions he accused of being a Nazi conspiring to have him killed.
At one point, Brinkema asked a defense attorney if Moussaoui knew what he was doing.
After a very long pause, attorney Alan Yamamoto told the judge, "He is facing the possibility of death or life in prison. He has told me that he understands that."
Once the guilty plea was accepted, Moussaoui appeared to switch gears, launching into one of the rambling tirades for which he has become famous. He complained he was misled by his attorneys, and insisted he was part of a conspiracy not related to Sept. 11.
"I will fight with every inch against the death penalty," he said.
He told the court that he was training at a Minnesota flight school to man 747s in order to fly a plane into the White House as part of a wave of post-Sept. 11 attacks. He hinted he would argue once again he had no role in the Sept. 11 attacks during the penalty phase.
Moussaoui's plea represents the first U.S. conviction in the Sept. 11 attacks (search), the worst act of terrorism on American soil.
Earlier on Friday, court-appointed defense attorneys filed an 11th-hour motion raising serious concerns about Moussaoui's mental state, just hours before he was due at the federal court house in Alexandria, Va.
The document, filed under seal, was titled, "Sealed Suggestion of Defense Counsel as to Defendant's Incompetence to Plead Guilty and for a Sentence of Death."
The motion may have contained results from a psychological examination ordered by Moussaoui's attorneys last fall; an expert found him to be insane, FOX News has learned.
Though questions about Moussaoui's mental fitness have dogged him since 2002, when he attempted to act as his own attorney, Judge Brinkema found him to be mentally competent after questioning him on Wednesday.
But some legal observers disagreed with that assessment.
"He has mouthed the necessary words, but his actions speak louder," said attorney Donna Newman, who represents "dirty bomb" suspect Jose Padilla. "It's clear he's not competent enough to understand what he is doing."
Legal scholar Jonathan Turley (search), who observed Moussaoui in court in 2002 when he was still acting as his own attorney, described the Al Qaeda operative as a "barking lunatic."
"The guilty plea is colored by lingering questions as to his competence," Turley said before the proceedings. "He spent two years in segregation, which cannot have helped his mental state. If he pleads guilty and is found competent, there will be lingering questions."
But former assistant U.S. attorney John Curran, who hailed the plea as a "critical win" for the government, said the competency bar for entering a plea was not very high, and Moussaoui met it.
"The standards for reaching a normal guilty plea were reached," he told FOX News. "Does he understand what he's doing and what the consequences are? That's a very low standard of competency."
Curran also expressed disappointment that there would be no criminal trial.
"It would be fantastic for the American people to see this play out," he said.
Friday's proceedings were by no means the end of the Moussaoui saga. A judge or jury, depending on what the two sides prefer, will oversee the sentencing phase, and the issue of Moussaoui's sanity is likely to come up again. In addition, Moussaoui may have the opportunity to question government witnesses himself.
It was still unclear why Moussaoui decided to plead guilty, considering the likelihood he will again argue he had no role in the attacks during sentencing.
Some legal observers wondered if Moussaoui was attempting to become a martyr for Al Qaeda (search). But he has repeatedly cited Islam's prohibition against suicide in arguing he had no part in the attacks that killed hundreds of innocent people and all 19 hijackers.
The guilty plea was not part of an agreement with prosecutors. Moussaoui faces the death penalty for charges of conspiracy to commit acts of terrorism, commit aircraft piracy, destroy aircraft and use weapons of mass destruction. Two other charges, conspiracy to destroy property and conspiracy to murder government employees, carry a lesser sentence.
Moussaoui is the only person charged by the U.S. government in the Sept. 11 attacks. The case against him was initially seen as an opportunity to showcase the fairness of the American justice system, but instead became a cautionary tale of the many ways in which due process and security needs can butt heads within the judiciary.
Much of the unpredictability of the case was due to Moussaoui himself. Though the 36-year-old had until now maintained he was not part of the Sept. 11 plot, he has been anything but shy about declaring his allegiance to Al Qaeda and hatred for America and Jews.
In numerous, handwritten filings that called to question his mental stability, Moussaoui has lashed out at his court-appointed defense team ("bloodsucker death team") and Judge Brinkema (a Nazi and the "furor" [sic]).
Moussaoui attempted to plead "no contest" and not guilty on two occasions in 2002; both times Judge Brinkema, skeptical he understood the consequences, refused to accept his plea.
On July 25, 2002, Moussaoui entered a not guilty plea, but quickly withdrew it after being told it would mean he was acknowledging a role in the attacks.
Moussaoui was detained on immigration violations by the FBI a month before the 2001 attacks after instructors at a Minnesota flight school became suspicious of him. He was initially thought of as the "20th hijacker," the person who would have been on board one of the four planes used to attack New York and Washington had he not caught the bureau's eye.
But interrogations of two Al Qaeda figures in U.S. custody — Sept. 11 mastermind Khalid Shaikh Mohammed (search) and Ramzi Binalshibh (search), who helped plan the attacks — have led intelligence officials to conclude he had no role in the Sept. 11 plot. There is evidence he was on Al Qaeda's payroll, but the cash may have been earmarked for use in future attacks.
Indeed, the Sept. 11 commission came to that conclusion as well, finding that Moussaoui instead may have been on some kind of retainer for a second wave of attacks.
"He is pleading guilty to a few counts based on the theory he was the 20th hijacker," Turley told FOXNews.com. "But U.S. officials have publicly said he was not the 20th hijacker."
Brinkema has tussled with the Justice Department over the defendant's access to testimony from Mohammed and Binalshibh, on several occasions indicating Moussaoui could be exonerated of the more serious charges against him.
Moussaoui was reportedly deemed too unstable by the Sept. 11 planners, according to officials familiar with the interrogations of Mohammed and Binalshibh.
In October 2003, after the U.S. government once again defied Brinkema's orders to produce direct access to the Al Qaeda chieftains, Brinkema responded by prohibiting the government from seeking the death penalty for Moussaoui.
But last September, the 4th Circuit reversed that order, making Moussaoui eligible for the death penalty again. Moussaoui's attorneys had argued that if the court would not permit testimony that could disprove the more serious charges against their client, then the death penalty must be taken off the table.
Earlier on Friday, an attorney for Moussaoui's mother said she feared years of solitary confinement had made her son desperate and mentally unstable.
"She feels like when she talks to him ... he's a totally different person than he was four years ago ... She doesn't feel he's mentally capable of declaring himself to be guilty. She feels he's just trying to end his solitary confinement," Randall Hamud told FOX News before Moussaoui's court appearance.
Aicha Moussaoui has said she believes the Justice Department sought to convict her son only out of vengeance.
Given the doubts about Moussaoui's role in the attacks, Turley said the court's acceptance of a guilty plea would be a "Pyrrhic victory" for the government.
"We're all going to watch as someone believed to be incompetent is going to take a cleaver to himself in the courtroom," Turley told FOXNews.com before Moussaoui entered the plea.
FOX News' Catherine Herridge contributed to this report.