The first Guantanamo war crimes trial began Monday with a not guilty plea from a former driver and alleged bodyguard for Osama bin Laden.

Salim Hamdan, a Yemeni, entered the plea through his lawyer at the U.S. Navy base in Cuba.

He is the first prisoner to face a U.S. war crimes trial since World War II.

Judge Keith Allred, a Navy captain, called a jury pool of American military officers into the courtroom and began reading them instructions. A minimum of five of the 13 officers must be selected for the trial.

Hamdan, a Yemeni, wore a khaki prison jumpsuit to the courtroom. The flowing white robe and headdress he wore at pretrial hearings was not cleaned in time for his trial, said Charles Swift, one of his civilian attorneys.

The trial is expected to take three to four weeks, with testimony from nearly two dozen Pentagon witnesses.

Hamdan was captured at a roadblock in Afghanistan in November 2001, allegedly with two surface-to-air missiles in the car. But his lawyers say he was merely a low-level driver and mechanic without any role in the al Qaeda conspiracy against the United States.

Hamdan was taken to Guantanamo in May 2002 and selected as one of the first inmates to face prosecution. His case has created repeated legal obstacles for the Pentagon including a Supreme Court ruling that struck down an earlier version of the tribunal system.

Allred began the proceedings Monday by indicating that he would not allow the government to use some of the evidence interrogators obtained from Hamdan during his detention in Afghanistan. Defense lawyers have argued those statements were tainted by "coercive" techniques and the fact that interrogators did not advise him of a right against self-incrimination.

Defense attorneys, who say Hamdan was merely a low-level bin Laden employee, have refused to say whether they are in negotiations over a possible guilty plea.

Julia Hall, senior counsel for Human Rights Watch's counterterrorism program, said Hamdan would not be a likely candidate for a plea deal because his case appears simple in comparison with others pending before the first U.S. war-crime tribunals since World War II.

"He has alleged direct associations with Osama bin Laden, he was not held in secret detention, and a federal court judge has ruled the commission can go forward," she said. "It seems obvious why he is a person that the government would want to test drive the military commissions on."

Meanwhile, U.S. Attorney General Michael Mukasey wants Congress to help figure out how to give Guantanamo Bay terror detainees their day in U.S. civilian courts.

A Supreme Court ruling last month "stopped well short" of detailing how foreign suspects will be allowed to challenge their detention, Mukasey said in draft excerpts of a speech he was to deliver Monday morning.

"In other words, the Supreme Court left many significant questions open," Mukasey said in the excerpts that were obtained by The Associated Press.

He called it "well within the historic role and competence of Congress and the executive branch to attempt to resolve them." Mukasey was speaking at the American Enterprise Institute, a Washington think-tank.

At issue is the June 12 ruling that struck down a provision of the Military Commissions Act of 2006 that denied Guantanamo detainees the right to file petitions of habeas corpus. Habeas corpus is a centuries-old legal principle, enshrined in the Constitution, that allows courts to determine whether a prisoner is being held illegally.

Mukasey wants lawmakers - and not federal district judges - to set the rules. It's unclear at best whether Congress could act by then, and any new laws setting such standards likely would be snarled in appeals for years.

Among the issues to be sorted out is how civilian judges might be allowed to review evidence against the prisoners. The Justice Department has fought for years to limit judicial review of evidence in these cases.

Mukasey noted that the Supreme Court acknowledged the hearings "could raise serious national security issues."

"The court recognized, and with good reason, that certain accommodations must be made to reduce the burden habeas corpus proceedings will place on the military and to protect sources and methods of intelligence gathering," Mukasey said.

Twenty Guantanamo detainees are facing charges, including five alleged plotters of the Sept. 11 attacks who were transferred here from secret CIA prisons in 2006. Prosecutors intend to charge as many as 80 inmates at the U.S. Navy base in southeast Cuba.