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Guantanamo detainee Omar Khadr has been transferred to Canada to serve out the remainder of an eight-year prison sentence for having killed an American soldier in a 2002 firefight in Afghanistan.

A detainee for a decade, Khadr had been held at the U.S. detention facility at Guantanamo since he was 16, making him one of youngest detainees ever held there.

The 26-year-old Canadian was among the more high profile detainees still held at Guantanamo because of the controversy surrounding his young age.

Khadr was 15 when captured in 2002 in Afghanistan shortly after a firefight that killed Army Sgt. First Class Christopher Speer.

Khadr's Egyptian father Ahmed Said Khadr had taken his family to live in Afghanistan in 1996. His father was believed to be a high level al Qaeda operative and Omar Khadr was said to have received training at an al Qaeda camp from the time he was 10 years old.

Khadr's brother Abdurahman was also a detainee at Guantanamo until 2003.

Human rights groups campaigned that Khadr should be treated as a child soldier and not as an adult enemy combatant.

In the end, U.S. military prosecutors proceeded with Khadr's prosecution before a military tribunal.

In October 2010, Khadr received an eight-year prison sentence after he pleaded guilty to having thrown a grenade that killed Speer, as well as to several war crimes charges.

Under the terms of the plea deal, Khadr was to serve a year of his sentence at Guantanamo before being transferred to Canada to serve out the remainder.

He was eligible to be returned to Canada last October, but the transfer was delayed as Canadian authorities reviewed the circumstances of his transfer.

A Defense Department statement today announced Khadr's transfer to Canada.

"The United States coordinated with the government of Canada regarding appropriate security and humane treatment measures,"the statement said. "In accordance with statutory reporting requirements, the administration informed Congress of its intent to transfer Khadr to Canada."

Canadian Public Safety Minister Vic Toews confirmed Khadr's return to Canada.

"Early this morning, convicted terrorist Omar Khadr was transferred to Canadian authorities at CFB Trenton," Toews said in a written statement. "He has been transferred from CFB Trenton to Millhaven maximum security prison at Bath, Ontario.

"I am satisfied the Correctional Service of Canada can administer Omar Khadr's sentence in a manner which recognizes the serious nature of the crimes that he has committed and ensure the safety of Canadians is protected during incarceration," he said.

The American Civil Liberties Union welcomed Khadr's transfer to Canada, but said Khadr should have never been held as a detainee at Guantanamo.

"We cannot forget his decade-long imprisonment in abusive U.S. custody," Jennifer Turner, a human rights researcher with the ACLU's Human Rights Program said. "Khadr was denied the fundamental rights of former child soldiers such as humane treatment, fair trial and other juvenile justice protections. His abhorrent Guantanamo experience should never have happened."

There are now 166 detainees still being held at Guantanamo. Khadr was the youngest detainee at Guantanamo and the last western detainee still being held at the facility.