A ninth prisoner has died at US concentration camp Guantanamo Bay, Cuba, the US military admitted on Monday, two days after the man was found unconscious in his cell.
The prisoner's name and nationality have not been officially released, but anonymous guards said he was from Yemen.
Experts have yet to determine what caused his death, said camp spokesman Captain Robert Durand.
"We haven't ruled in or out anything," he said. "There is no apparent cause, natural or self-inflicted."
But Mr Durand said the prisoner was a former hunger striker who had returned to 95 per cent of his ideal weight.
Six of the previous deaths since the camp's January 2002 opening were declared suicides while the US military claims the other two were natural.
Former prisoner Moazzam Begg, who is now director of advocacy group Cageprisoners, said the death underscores the bleak outlook for the 167 men held at Guantanamo.
"Almost 11 years since the camp opened, few people are concerned whether the men are imprisoned or free, whether they live or die," said Mr Begg, a British citizen who was released in 2005.
The latest death occurred in Camp 5, a section of the prison used mostly to hold prisoners who have broken detention centre rules.
More than half of prisoners at Guantanamo have been cleared for release, but the US claims that it is has been unable to find stable countries to transfer them to.
Wells Dixon, a lawyer who has represented a number of Guantanamo prisoners, said the sense of despair among prisoners seems to have worsened since the Supreme Court announced in June that it would not review the way courts were handling individual appeals.
"The mood is very dark," Mr Dixon said. "Many of them have lost any hope that they are ever going to be released, regardless of their status."
Mr Durand admitted that the man who died on Saturday had not been charged with any crime and was not designated for prosecution.