The youngest Guantanamo prisoner, Omar Khadr, who was a 15-year-old fighting in Afghanistan when captured in 2002, was sent to finish his sentence in his native Canada.
A military plane carrying Khadr, who pleaded guilty to killing a US soldier and admitted links to al-Qaeda, left the US naval base on Saturday morning, the Toronto Star newspaper and Canadian Broadcasting Corporation said.
Vic Toews, Canada's public safety minister, confirmed the return of Khadr to Canada at a hastily arranged media briefing in Winnipeg, Manitoba.
A US war crimes tribunal in 2010 sentenced Khadr, now 26, to 40 years in prison, although he was expected to serve just a few more years under a deal that included his admission he was an al-Qaeda conspirator who murdered a US soldier.
Khadr pleaded guilty in 2010 to charges that included conspiring with al-Qaeda to commit terrorist acts, making roadside bombs to target US troops in Afghanistan, spying on American military convoys and providing material support for terrorism.
'Family of terrorism'
Khadr was eligible to return to Canada from Guantanamo Bay last October under terms of a plea deal. But Canadian Prime Minister Stephen Harper's Conservative government had long refused to request the return of Khadr.
The reluctance was partly due to suspicions about the Khadr family, which has been called "the first family of terrorism".
"His head is spinning a bit and it's going to be a real adjustment for him, but at the same time he is so happy to be home,'' John Norris, Khadr's Canadian lawyer, said after speaking with his client.
"He can't believe that it is finally true. He simply can't. For very good reason he was quite fearful that the government would not follow through on its word and he's pinching himself right now not believing that this government has finally kept its word,'' he said.
Norris said Khadr would be eligible for parole as early as the summer of 2013. He said Khadr's return to Canada comes 10 years too late.
Vic Toews, Canada's public safety minister, said the US government initiated Khadr's transfer and suggested that Canada had little choice but to accept him because he is a Canadian citizen.
It will be up to Canada's national parole board to release him, Toews said.
"I am satisfied the Correctional Service of Canada can administer Omar Khadr's sentence in a manner which recognises the serious nature of the crimes that he has committed and ensure the safety of Canadians is protected during incarceration,'' Toews said.
Norris said it is regrettable that the minister is trying to influence the parole board.
"Most of what he has said there is simply not true. It's part of the stereotype of Omar that this government has been disseminating from the beginning,'' Norris said.
The US Defence Department confirmed the transfer in a statement and said 166 detainees remain in detention at Guantanamo Bay.
Khadr was 15 years old when captured in Afghanistan in 2002. He was the first person since World War Two to be prosecuted in a war crimes tribunal for acts committed as a juvenile.
Khadr was taken to Afghanistan by his father, a senior al-Qaeda member who apprenticed the boy to a group of bomb makers who opened fire when US troops came to their compound.
Khadr was captured in the firefight, during which he was blinded in one eye and shot twice in the back.