Lawyers for Omar Khadr called on U.S. authorities Monday to dismiss a murder charge against the Canadian, saying a newly revealed eyewitness account that had been covered up by the Pentagon casts doubt on the official version of events.

Khadr, now 21, is charged with hurling a grenade that killed American Sgt. Christopher Speer during a firefight in Afghanistan in 2002. He's been in custody at a U.S. military prison in Guantanamo Bay, Cuba, since then.

According to the original U.S. military version of events, Khadr ambushed American soldiers with a grenade following a four-hour fight at a mud compound in Afghanistan.

Pentagon officials later backtracked slightly after it was revealed nobody witnessed Khadr throw the grenade. Pentagon officials said an eyewitness wasn't needed, because Khadr was the only al-Qaeda fighter left alive and the only person who could have thrown the grenade.

However, a classified document, inadvertently released to reporters at the military prison by a Pentagon official Monday, provides a different eyewitness account of the events.

A U.S. soldier at the battle said in sworn testimony that two al-Qaeda fighters were alive after the fatal grenade attack.

The unidentified soldier says he killed the first al-Qaeda fighter before spotting Khadr, whom he said was wounded, on his knees and facing away from him. For reasons he does not go into, he says he shot him in the back twice.

The Pentagon says American soldiers fired on Khadr in self-defence after he tried to attack them.

Khadr's military lawyer Lt.-Cmdr. Bill Kuebler suggests that the U.S. military may have been involved in a coverup.

"The U.S. government had a problem on its hands when it found that it had a 15-year-old Canadian on its hands with two gaping bullet holes in his back that had been facing away from the fight," said Kuebler.

Kuebler hopes the eyewitness account introduces enough reasonable doubt that Judge Peter Brownback will have to dismiss the murder charge against Khadr.

The lawyers argue that Khadr's alleged offences occurred in a combat setting, and therefore should not be considered war crimes committed by an irregular "enemy combatant."

They're asking a military judge to drop the charges, since Khadr was only 15 when he was captured.

If convicted, Khadr faces a maximum sentence of life in prison.

With files from the Canadian Press