Majid Khan, a Pakistani who disclosed how he was tortured by the CIA after the September 11, 2001, attacks on the US, has been transferred from the Guantanamo Bay prison in Cuba to Belize, the Pentagon said on Thursday.
Khan admitted in 2012 to conspiring with members of Al Qaeda responsible for the 2001 attacks to commit murder, and providing material support for terrorism and spying.
He had since become a government witness, US officials said.
Khan was captured in Pakistan and previously held at an unidentified CIA black site from 2003 to 2006.
Thirty-four detainees, down from a peak population of 800, remain at the Guantanamo Bay, with 20 already eligible for transfer, according to US officials.
Defence Secretary Lloyd Austin notified US legislators about his intent to transfer Khan last year, the Pentagon said.
"The United States appreciates the willingness of the government of Belize and other partners to support ongoing US efforts focused on responsibly reducing the detainee population and ultimately closing the Guantanamo Bay facility," it said.
In a 39-page statement Khan read to a military sentencing commission in 2021, he described being hung from a beam by his hands for days, naked except for a hood over his head at the CIA site.
Guards would "throw ice water on my naked body every hour or two and placed a fan to blow directly on me”, he said.
Khan told of being beaten, subjected to the simulated drowning technique called waterboarding and sexually abused.
He also said he had been deprived of sleep and food, kept isolated and shackled in a cell with music blaring 24 hours a day.
This went on for three years, from the time of his arrest in Karachi, Pakistan, in 2003 until he was transferred to Guantanamo Bay, Khan said.
The detention camp at the Guantanamo Bay US naval base was opened under Republican President George W Bush in 2002.
Former president Barack Obama, a Democrat who succeeded Mr Bush, whittled down the number, but his effort to close the prison was blocked largely by Republican opposition in Congress.