A U.S. government official's eyewitness account of a firefight that killed a U.S. soldier in Afghanistan could derail the Bush administration's plans to put a former child soldier on military trial, defense attorneys said Thursday.

Omar Khadr's lead attorney, Navy Lt. Cmdr. William Kuebler, said the evidence could prove the Guantanamo detainee does not merit a designation as an "unlawful enemy combatant," which is required for him to face trial on this U.S. Navy base.

The evidence was revealed by U.S. military prosecutors to defense attorneys on Tuesday — five years after the Canadian teenager was detained. The attorneys couldn't discuss the classified evidence or identify the witness, but Kuebler was clearly angry over the limited disclosure to defense attorneys.

"How much other exculpatory evidence is out there behind the black curtain that we can't see?" Kuebler told reporters.

The witness described a 2002 firefight in Afghanistan in which Khadr, then 15, allegedly threw a grenade that killed Sgt. 1st Class Christopher Speer, a Delta Force commando, at an al-Qaida compound, the defense attorneys said.

The presiding judge, Army Col. Peter Brownback, postponed a decision Thursday on whether Khadr can be tried by the military as an "unlawful enemy combatant."

Khadr, the Toronto-born son of an alleged al-Qaida financier, answered the judge's questions politely, saying "Yes, sir" and "Yeah," and did not enter a plea to charges including murder, conspiracy and spying. He appeared for the hearing with a short beard and the white prison uniform reserved for the most compliant detainees.

Kuebler said the prosecutors were legally obligated to share the new evidence and criticized the government for not making the information available sooner as defense attorneys prepare for the first U.S. war-crime trials since the World War II era.

Kuebler also complained that defense lawyers have been barred from interviewing an FBI agent who deposed Khadr and a U.S. Army officer who led the raid on the alleged al-Qaida compound, even though both are at Guantanamo.

The lead prosecutor, Marine Corps. Maj. Jeffrey Groharing pleaded for an opportunity to present evidence against Khadr. He said the government had flown in witnesses and has a videotape that shows Khadr planting land mines. But Brownback stood by his postponement, and set deadlines of Dec. 7 and Jan. 11 for attorneys to file motions. No trial date was set.

Only unlawful enemy combatants can be tried by the military commissions, according to the Military Commissions Act, approved by Congress and signed into law by U.S. President George W. Bush last year. The law sought to legitimize war-crime tribunals that had been declared unconstitutional by the Supreme Court.

Brownback dismissed the charges against Khadr in June because he had not been classified as an "unlawful" enemy combatant. A hastily created military appeals court then ruled that Brownback had authority to attach that label himself.

But the judge said Thursday that there was no need to immediately address Khadr's status because defense lawyers have not formally requested clarification. Brownback also dismissed Kuebler's request that he remove himself for lack of impartiality.

The defense team, meanwhile, has challenged the appeals court ruling and said they were not conceding that the military court has jurisdiction.

The judge declined to answer questions about whether the tribunal system is constitutional, but acknowledged criticism from the Pentagon for dismissing Khadr's charges in June.

"The DoD (Department of Defense) people, they didn't like what I wrote," said Brownback.

Critics said Thursday's events reflect flaws in a system that has yet to produce a trial.

"It does question the wisdom of bringing everyone down here for a proceeding with a status and procedure that is still unknown and unknowable," said Jumana Musa, advocacy director of Amnesty International.

Khadr is one of three Guantanamo detainees facing charges under the Military Commissions Act. The military plans to prosecute as many as 80 of the 320 men at Guantanamo. But the Supreme Court may have other ideas. A challenge to the reconstituted system is pending and detainee lawyers have asked the justices to guarantee they can challenge their confinement in U.S. civilian courts.

Separately Thursday, a study released by a Seton Hall University professor and his son found that only 21 detainees of 517 about whom the U.S. government has provided unclassified information were captured on a battlefield.

The law professor, Mark Denbeaux, said the report based on the Pentagon information and a recent study by West Point refuted U.S. claims that many detainees were caught fighting against American and coalition forces.

A Guantanamo spokesman, Army Lt. Col. Ed Bush, said in response to the claims: "We encourage people to come for themselves and form their own opinions."