Omar Khadr is serving time in a Canadian prison for a crime that didn’t exist when he was captured by the U.S. in Afghanistan in 2002, his lawyers argued in a federal court appeal filed in Washington Friday.

This is the first legal challenge in the U.S. since the 27-year-old Khadr accepted a Pentagon plea deal three years ago in Guantanamo, admitting he threw a grenade that fatally wounded U.S. Delta Force soldier Christopher Speer during a July 2002 firefight.

Khadr, who is currently being held in a maximum security federal institute in Edmonton was convicted of five war crimes, including “murder in violation of the laws of war’’ for Speer’s death and sentenced to eight years. He was transferred to Canadian custody from Guantanamo last year.

But Khadr’s Pentagon-appointed lawyer Sam Morison writes in his Washington appeal that the conviction must be overturned and Khadr released immediately since those war crime offences only were established as U.S. law in 2006 and Khadr should not have been prosecuted retroactively.

Khadr’s court challenge comes in the wake of other high-profile cases where Guantanamo convictions were overturned in similar circumstances.

The U.S. Court of Appeals for the District of Columbia’s three-judge panel cleared Yemeni Salim Hamdan, Osama bin Laden’s former driver and one of Guantanamo’s most famous prisoners, in an October 2012 ruling that stated he could not be convicted of “material support for terrorism,” which was not a war crime at the time Hamdan chauffeured the Al Qaeda leader.

War crimes trials for Guantanamo detainees are governed under the Military Commissions Act, which was established by the Bush administration following the Sept. 11, 2001 attacks and amended once U.S. President Barack Obama came into power.

Former Australian Guantanamo detainee David Hicks also filed an appeal this week to overturn his conviction. Hicks pleaded guilty in March 2007 to providing material support for terrorism before a military court at the U.S. naval base in Guantanamo Bay, Cuba.

Both Hicks and Hamdan were released shortly after they were repatriated to Yemen and Australia.

Pentagon spokesperson Todd Breasseale said that it was premature for the Department of Defense to comment on the case. “The response to Mr. Khadr’s arguments will come when the government files its brief,” he said Friday.

Khadr’s challenge will be the first dealing with murder and attempted murder convictions and Morison says should be addressed quickly since Khadr remains in custody in a maximum security institution serving the remainder of an 8-year sentence.

“It’s urgent because he was wrongfully convicted,” said Morison, who is expected to travel to Edmonton next week where Khadr is detained. “After more than 10 years he’s entitled to get on with his life.”

But part of Khadr’s plea deal was an agreement to not appeal his conviction. Morison said he plans to argue that if the underlying acts were not war crimes, then his guilty plea and all its conditions must also be thrown out.

“Basically the judge had no authority to accept his guilty plea in the first place,” Morison said.

In the written appeal filed with the U.S. Court of Military Commission Review, Morison and Khadr’s Canadian lawyer Dennis Edney write that Khadr’s guilt was based on his own claims which were “obtained through a series of coercive, uncounseled, and abusive interrogations that began as soon as he regained consciousness, approximately one week after the firefight.”

Morison is also arguing that the court must consider Khadr’s age and circumstances of detention. “The government forfeits the moral authority to prosecute the target of its abuse,” Morison writes. “If the government’s choice to detain a gravely injured 15-year-old child as an adult in a maximum security prison and to subject him to a systematic regime of physical and psychological abuse does not shock the conscience, then nothing does.”

The appeal describes how Khadr was interrogated while shackled to a stretcher in Bagram, and details of the Pentagon’s immunity deal with Khadr’s main interrogator, Joshua Claus, who was implicated in the death of another detainee at Bagram.

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The Toronto-born Khadr was 15 when he was shot and captured following the battle with U.S. Special Forces. He was taken to the U.S. base in Bagram, Afghanistan, before his transfer to Guantanamo three months later. Although Guantanamo’s chief prosecutor said Khadr’s age was taken into consideration during his sentencing, it was not a factor in his detention or prosecution.

Morison said Khadr agreed to the plea deal so he could return to Canada.

“The reason he pled guilty is because it was the only way he was going to get out of Guantanamo Bay,” Morison said in an interview. “Unlike any other criminal justice system in the world, going to trial wasn’t really a meaningful option because even if he had gone to trial, and was acquitted, he knew the government could have held him indefinitely.”