The U.S. Supreme Court turned away on Monday a Canadian former Guantanamo Bay detainee's bid to vacate his convictions for the 2002 murder of an American soldier in Afghanistan and other crimes he committed at age 15 to which he later pleaded guilty.

The justices declined to hear an appeal by Omar Khadr, now age 37, of a lower court's refusal to hear his case on the grounds that he had waived his right to appellate review as part of a 2010 plea agreement before a U.S. military commission.

Khadr was one of the youngest prisoners held at the detention facility at the U.S. naval base in Cuba. Khadr pleaded guilty in exchange for an eight-year sentence and transfer to a Canadian prison. He was granted bail in 2015 and completed his sentence in 2019 as he continued to pursue dismissal of his U.S. convictions.

OMAR KHADR: EX-GITMO DETAINEE WHO KILLED US SOLDIER RECEIVES MILLIONS FROM CANADA

He was taken to Afghanistan by his father, a senior al Qaeda member who apprenticed his son to a group of bomb makers who opened fire when U.S. troops came to their compound in 2002. During the firefight, Khadr, 15, threw a hand grenade that killed Sergeant Christopher Speer, a U.S. Army medic. Khadr was gravely wounded - shot twice - when he was captured by U.S. forces.

In 2007, Khadr was charged under a 2006 U.S. law called the Military Commissions Act with five crimes including murder and attempted murder in violation of the law of war, and providing material support to terrorism. He was 24 when he pleaded guilty.

In 2012, a federal appeals court in a separate Guantanamo Bay detainee's case issued a decision with potential implications for Khadr. The U.S. Court of Appeals for the District of Columbia Circuit ruled that defendants could not be charged under the Military Commissions Act for certain crimes that occurred prior to the law's adoption in 2006.

Despite having agreed to waive his right to appellate review, Khadr appealed to the D.C. Circuit. Attorneys for Khadr argued that his convictions, which were based on actions he took in 2002 before Congress passed the statute, violated the U.S. Constitution's ban on criminalizing conduct after it has occurred.

The D.C. Circuit rejected Khadr's appeal because of his waiver of appellate review.

At issue in Khadr's petition to the Supreme Court was whether he is bound by his agreement to waive his right to appeal, not whether his convictions should be immediately vacated.

Khadr's attorneys told the Supreme Court that although Khadr had agreed to waive his right to appeal, he had not actually filed the paperwork to finalize the waiver when the D.C. Circuit issued its ruling establishing a new legal standard favorable to Khadr's case.

President Joe Biden's administration had urged the justices to turn away Khadr's appeal.

Khadr's plea deal came in a case that made the United States the first nation since World War Two to prosecute a defendant in a war crimes tribunal for acts allegedly committed as a juvenile. Khadr's lawyers had argued unsuccessfully at the time that he was a child soldier who should be rehabilitated rather than prosecuted in a military tribunal.

Canada formally apologized to Khadr in 2017 "for any role Canadian officials may have played in relation to his ordeal abroad and any resulting harm" and paid out $7.83 million in compensation.

The United States opened the Guantanamo detention facility for foreign terrorism suspects in 2002, months after U.S. forces invaded Afghanistan in the aftermath of the Sept. 11, 2001, attacks on the United States by al Qaeda militants who were harbored by the country's Taliban leaders. The Taliban returned to power in Afghanistan in 2022 after Biden withdrew U.S. forces.