A new US appeals court has begun hearing the case of a Canadian accused of killing a US soldier in Afghanistan.
Canadian Omar Khadr was accused of killing a US soldier in 2002
The US government has gone to the Court of Military Commission Review to appeal against a decision by military judges in June to throw out Omar Khadr's case.
The case collapsed because US military authorities had failed to designate him an "unlawful" enemy combatant.
They had sought to make Mr Khadr one of the first Guantanamo Bay inmates to be tried before a military commission.
Critics have questioned the legality of the new appeals court, which was quickly set up and staffed after the US government appealed against the judges' ruling.
But in opening discussions, the military judges did not appear to accept Mr Khadr's attorneys' argument that the court had been improperly formed.
The appeals court is expected to rule within a month on whether Mr Khadr should have to face a military commission.
The US authorities hope the appeals court's decision will end the legal limbo which has paralysed the new tribunal system since June.
The issue stems from the fact that under a new system of military justice approved by Congress last year, detainees facing trial must be designated "unlawful enemy combatants".
When they were assessed years earlier they were described only as "enemy combatants".
The word "unlawful" did not appear, giving the new tribunals no jurisdiction, according to the military judges who heard Mr Khadr's case.
The Pentagon argues there is no material difference between the two terms and that it is a matter of semantics.
If the appeals court rules against the government, it is possible that review tribunals will have to reassess some 520 prisoners designated simply as "enemy combatants".
Mr Khadr, now 20, was only 15 when he was captured in Afghanistan, and was accused of killing a US soldier during a battle at a suspected al-Qaeda base in 2002.
His lawyers argue that he cannot be labelled as a terrorist until the charges against him are proven in court.
The case against another Guantanamo Bay detainee, Yemeni Salim Ahmed Hamdan, was also dropped because of the failure of US military officials to call him "unlawful".
Salim Ahmed Hamdan has been accused of being al-Qaeda leader Osama Bin Laden's driver and bodyguard. Mr Hamdan says he was just a driver and not an al-Qaeda member.
He had appeared in court charged with murder, attempted murder, conspiracy and providing support for terrorism.