They met behind closed doors for about 45 minutes before recessing until Tuesday morning.
Salim Hamdan, a Yemeni held here since May 2002, faces a maximum sentence of life in prison if convicted of conspiracy and supporting terrorism.
In closing arguments, prosecutors said Hamdan's service to the al-Qaida chief over five years in Afghanistan helped his boss execute terrorist plots including the Sept. 11, 2001, attacks on New York and Washington.
"He is an al-Qaida warrior," Justice Department prosecutor John Murphy said, pointing to the detainee who wore a white robe and a tan sports coat.
Defense lawyers counter that Hamdan was merely a member of bin Laden's motor pool. His Pentagon-appointed attorney, Navy Lt. Cmdr. Brian Mizer, said in closing arguments that his client never joined al-Qaida and had no part in planning attacks.
"If every garage mechanic and driver knew the details and was involved in the planning of the attack, it never would have happened," Mizer said.
The judge, Navy Capt. Keith Allred, told the jurors that four of the six must find Hamdan guilty "beyond a reasonable doubt" to convict him.
Even if they find him innocent, Hamdan may not be released. The military retains the right to hold "enemy combatants" considered a threat to the United States - even those cleared of charges by the tribunals.
A guilty verdict would be followed immediately by a sentencing hearing at Guantanamo. The sentence would have to be approved by the Pentagon official who oversees the tribunal system, Susan Crawford, and the verdict would be reviewed automatically by a special military appeals court in Washington.
Defense attorneys said they would anticipate an additional appeal to a U.S. civilian court in Washington.
Most of the evidence against Hamdan came from U.S. federal and military agents who interrogated him in Afghanistan and Guantanamo, including one who said that Hamdan swore an oath of allegiance to the terrorist leader and expressed "uncontrollable enthusiasm" for his cause.
Allred urged jurors to evaluate that evidence in light of a U.S. policy that prevented interrogators from advising Hamdan of a right to avoid incriminating himself. Military prosecutors have contested whether detained "enemy combatants" have any constitutional rights.
"You must decide the weight and significance, if any, such statements deserve," Allred told the jurors, who were chosen by the Pentagon and flown to the base in southern Cuba for the case.
One of Hamdan's civilian attorneys, Joseph McMillan, was warned by the judge against threatening jurors as he began to describe the international scrutiny on the trial.
"It is attention that has significance for the war on terror," McMillan said.
Hamdan left the courtroom early because he was being allowed a rare phone call with his family, Allred said.
Hamdan was captured at a roadblock in southern Afghanistan in November 2001 with two surface-to-air missiles in the car.
Prosecutors accused him of transporting weapons for al-Qaida and evacuating bin Laden to safety after learning he was about to launch terrorist "operations," including the Sept. 11 attacks.
The military has charged 21 of the roughly 265 men held at Guantanamo Bay on suspicion of terrorism or links to al-Qaida or the Taliban. Military prosecutors say they plan trials for about 80 inmates.
So far, only one Guantanamo inmate has been convicted. Australian David Hicks reached a plea agreement that sent him home to serve a nine-month prison sentence.
