GUANTANAMO BAY NAVAL BASE, Cuba — Military judges dismissed charges yesterday against a Guantanamo detainee accused of chauffeuring Osama bin Laden and another who allegedly killed an American soldier in Afghanistan, throwing up roadblocks to the Bush administration’s attempt to try terror suspects in military courts.

In back-to-back arraignments for Salim Ahmed Hamdan of Yemen and Canadian Omar Khadr, the American military’s cases against the alleged Al Qaeda figures dissolved because, the two judges said, the government had failed to establish jurisdiction.

They were the only two of the roughly 380 prisoners at Guantanamo charged with crimes, and the rulings stand to complicate efforts by the American government to try other suspected Al Qaeda and Taliban figures in military courts.

Mr. Hamdan’s military judge, Navy Captain Keith Allred, said the detainee is “not subject to this commission” under legislation passed by Congress and signed by President Bush last year. Mr. Hamdan is accused of chauffeuring Mr. bin Laden’s and being the Al Qaeda chief’s bodyguard.

Defense attorneys argued that the new Military Commissions Act, written to establish military trials after the U.S. Supreme Court last year rejected the previous system, is full of problems.

The judges agreed that there was one problem they could not resolve — the new legislation says only “unlawful enemy combatants” can be tried by the military trials, known as commissions. But Messrs. Khadr and Hamdan had previously been identified by military panels only as enemy combatants, lacking the critical “unlawful” designation.

The surprise decisions do not spell freedom for the detainees, who are imprisoned here along with about 380 other men suspected of links to Al Qaeda and the Taliban. Mr. Khadr was 15 when he was captured after a firefight in Afghanistan in 2002 in which he allegedly killed an American soldier and was wounded himself. He is now 20.

Mr. Khadr, appearing in the courtroom with a beard and wearing an olive-green prison uniform, seemed uninterested when the judge, Army Colonel Peter Brownback, threw out the case. Mr. Khadr focused on his own image on a computer screen that showed a live TV broadcast of the proceedings.

The chief of military defense attorneys at Guantanamo Bay, Marine Colonel Dwight Sullivan, said the dismissal of the case against Mr. Khadr could spell the end of the war-crimes trial system hurriedly set up last year by Congress and Mr. Bush after the Supreme Court threw out the previous system.

But legal experts said Colonel Brownback apparently left open the door for a retrial for Mr. Khadr, and that the Defense Department can possibly fix the jurisdictional problem by holding new “combat status review tribunals” for any detainee headed to trial.

Colonel Sullivan said the dismissal has “huge” impact because none of the detainees held at this isolated military base in southeast Cuba has been found to be an “unlawful” enemy combatant.

“It is not just a technicality; it’s the latest demonstration that this newest system just does not work,” Colonel Sullivan told journalists. “It is a system of justice that does not comport with American values.”

The Military Commissions Act specifically says that only those classified as “unlawful” enemy combatants can face war trials here, Colonel Brownback noted.

The distinction is important because if they were “lawful,” they would be entitled to prisoner of war status, which under the Geneva Conventions would entitle them to the same treatment under established military law that American soldiers would get. A Pentagon spokesman said the issue was little more than semantics.

Navy Commander Jeffrey Gordon told the Associated Press said the entire Guantanamo system was set up to deal with people who act as “unlawful enemy combatants,” operating outside any internationally recognized military, without uniforms, military ranks or other things that make them party to the Geneva Conventions.