The United States and several other countries have committed human rights violations against a Saudi man held in the Guantanamo prison and accused of organising the bombing of the USS Cole in the port of Aden in 2000, a U.N. report said.
Abd al Rahim al Nashiri could face the death penalty if convicted for his alleged role in the attack.
The United Nations working group on arbitrary detention said he had been arbitrarily detained for more than 20 years and it voiced concern about his physical and mental well-being.
The United States has accused Nashiri of organising the attack on the USS Cole by two suicide bombers in small boat as the warship was moored in the Yemeni port of Aden. Seventeen sailors were killed and dozens wounded.
The al Qaeda Islamist militant group claimed responsibility for the bombing, which took place 11 months before the 9/11 attacks on the United States.
Washington did not respond to the U.N. body's report. The U.S. Defense Department, which operates the prison at the U.S. naval station at Guantanamo Bay in Cuba, did not respond to a request for comment.
Nashiri has been held at Guantanamo since 2006. The working group said he suffered discrimination on the basis of his nationality and his religious beliefs as a Muslim and said he had been deprived of fair trial guarantees.
Sylvain Savolainen, a lawyer for Nashiri based in Geneva, said the outcome was important and he would keep fighting "to put an end to the infamy he has suffered, and still suffers".
The U.N. document focuses on the Guantanamo detention but also refers to seven other countries - Afghanistan, Lithuania, Morocco, Poland, Romania, Thailand, United Arab Emirates - which allegedly transferred or detained him between 2002-2006.
At times, he was held in Central Intelligence Agency "black sites", a U.S. congressional report said, and was subjected to interrogation techniques which critics call torture.
The U.N. document said that the allegations of torture "stand unrefuted".
Afghanistan, the UAE, Thailand did not respond to the U.N. body; Lithuania's response was late and discounted; Poland said it had already paid "just satisfaction" to the applicant after a 2014 judgment; Romania contested the allegations and referred to a past judgment; Morocco rejected the allegations, saying Nashiri was never registered there.
The U.N. group's decisions carry political weight but are non-binding.
The Guantanamo prison was set up by then U.S. President George W. Bush in 2002 to house foreign militant suspects following the 9/11 attacks. President Joe Biden has said he wants to close the facility but has not presented a plan for doing so.
