Human rights lawyers in the US have criticised the Pentagon's decision to send a detainee at the Guantanamo Bay prison camp back to his native Egypt.
Most Guantanamo prisoners have been held years without charge
Sami al-Laithi was transferred at the weekend after being held without charge at the camp for more than three years.
Lawyers for the New York-based Center for Constitutional Rights said there was a risk he could be jailed or tortured in Egypt.
The Pentagon said Egypt had guaranteed he would be treated humanely.
According to the US State Department's annual report on human rights, torture of prisoners by Egyptian security forces is common.
The decision to transfer Mr Laithi came about four months after he was cleared of being an "enemy combatant", the Center for Constitutional Rights said.
His lawyer was not notified in advance of the move, the group said, and it is not known if Mr Laithi is being held in detention in Egypt.
His lawyer had filed a motion last month seeking to block his transfer to Egypt on the grounds he could face detention or abuse. A similar motion was turned down in August.
Lawyer Barbara Olshansky, of the Center for Constitutional Rights, said the decision to return Mr Laithi showed a "willing disregard" for human dignity by the US.
"You have someone who the US has decided is no longer an enemy combatant and is not dangerous, and we sent him back to a country that regularly puts people in secret detention and tortures them," she said.
Pentagon spokesman Cmdr Flex Plexico rejected the criticism, saying the US did not send individuals to countries where there was a likelihood of persecution.
The US had received "appropriate assurances" from Egypt that Mr Al Laithi would "continue to be treated humanely" on his return, he said.