TORONTO — U.S. forces shot Omar Khadr twice in the back before the young Canadian terror suspect allegedly killed one of their commandos during a 2002 firefight in Afghanistan, according to one witness.

The unidentified U.S. fighter said Khadr was hit by shrapnel and was facing away from the firefight when he was shot inside an al Qaeda compound.

The statement came on a day Khadr's lawyers were pushing to have all the charges against him dropped.

The legal representatives of the Canadian being held in Cuba's Guantanamo Bay appeared at the military tribunal on Monday.

They argued for the dismissal of the charges based on the following criteria:

• The fact that the tribunal itself was not established until after the alleged crimes occurred.
• Their belief that the crimes occurred during a legitimate military exchange.

Khadr, now 21, was 15 when he was captured by U.S. forces in July 2002 while fighting against the Americans in Afghanistan. He stands accused of lobbing a grenade that killed a U.S. Special Forces commando.

The tribunal heard arguments Monday on whether it has the right to try Khadr for his alleged crimes. He is accused of murder, conspiracy and providing material support to terrorism, and could face up to life in prison if convicted.

The eyewitness also said a second enemy fighter was alive inside the compound when he entered. Khadr's defence lawyers said this casts doubt on the U.S. government's conclusion that Khadr threw the grenade.

"It does raise questions as to basic facts of the government's case," said Khadr's lead defence lawyer, U.S. Navy Lt.-Cmdr. William Kuebler.

The judge, army Col. Peter Brownback, did not immediately issue a ruling.

Khadr was not shackled during his appearance Monday. He wore a white prison jumpsuit and had a short trimmed beard, and kept his eyes on the floor during much of the appearance, The Associated Press reports.

The tribunal heard from Rebecca Snyder, one of Khadr's defence lawyers, that Khadr can't be tried for murder as a war crime because the offence occurred during a battle and should be classified under traditional laws of war.

"Soldiers are not protected targets," she argued. "That is part of what war is about, killing soldiers."

Earlier Monday, Khadr's Canadian lawyer Dennis Edney told CTV's Canada AM the defence had a number of concerns.

"We have a number of motions before the judge," lawyer Edney said.

"One, of course, is that Omar Khadr be designated a child soldier. The other is we're challenging the definition of terrorism, particularly Omar Khadr being detained as a terrorist on a tobacco field. Essentially, we're challenging all the various indictments against Mr. Khadr."

The lead prosecutor in the tribunal, Marine Corps Maj. Jeffrey Groharing, said Khadr wasn't following accepted ruled of legitimate warfare and therefore should be tried by the special court.

He pointed out that Khadr conducted surveillance in civilian clothing, and lived with women and children at a compound where the fighting took place.

"The accused and the terrorists he was working with did not belong to a legitimate army, they belonged to al Qaeda,'' Groharing said.

Col. Peter Brownback, who is presiding over the hearing, did not immediately issue a ruling.

Edney said Monday's court proceedings in Cuba are an "extremely meaningful" part of the trial process, but he questioned whether the outcome would be favourable towards his client.

He said the Canadian government has let his client down, and must share in taking responsibility for his current plight.

"The Canadian government stands out amongst most other civilized countries throughout the world. It has not only failed to criticize Guantanamo Bay as a blight on the rule of law, it has not said a single word on behalf of Omar Khadr," Edney said.

"And that stands in contrast to the language it uses when criticizing the Chinese government's human rights record. So Canada will not say a single word of criticism about Omar Khadr to the Americans, and by not doing so, one then would suggest Canada is implicated with the Americans on their view of the law and the on war on terrorism."

Amir Attaran, a law professor at the University of Ottawa, called the tribunal a "kangaroo court"

"He was a child soldier and there has not been in the Western world a prosecution against a child soldier since the 18th century. So this is something really quite perverse that the United States court is doing, and that's the point of his lawyers, it should be thrown out of the court, done with," Attaran told Canada AM.

Edney said his first challenge was just to meet with his client. He spent much of yesterday waiting to meet with him, but was eventually denied the opportunity "because of security and various other strange reasons."

He said it was difficult to discuss their strategy because security restrictions prohibit Khadr from seeing all the evidence against him.

Kuebler told The Associated Press that the trial, if it proceeds, could set a dangerous precedent.

"The best case scenario is the judge does the right thing and dismisses the charges on the grounds that Congress did not intend this system to apply to a minor,'' Kuebler said.

He said the judge would be the first in western history to preside over a trial for alleged war crimes committed by a child.

The U.S. Defence Department maintains that the tribunal system established by the Bush administration in 2006 is the most appropriate venue to prosecute Khadr. He has been described as an "enemy combatant" by the U.S.

Khadr is accused in court documents of travelling through Afghanistan and Pakistan with his father from the time he was 10 years old. His father, an alleged al Qaeda financier, took him with him to meet with militant leaders such as Osama bin Laden, the documents allege.