A prisoner has died at the U.S. Navy base in Guantanamo Bay, Cuba, the U.S. military said Monday, two days after the man was apparently found unconscious in his cell at the isolated, high-security prison.

The prisoner, whose name and nationality were not released, was found by guards on Saturday and taken to a base hospital, where he was declared dead "after extensive lifesaving measures had been performed," the U.S. military's Southern Command said in a brief statement.

He was the ninth prisoner to die at the base since it was opened in January 2002 to hold men suspected of terrorism or links to al-Qaida and the Taliban. The military has said two of those deaths were by natural causes and six were declared suicides.

A prison spokesman, Navy Capt. Robert Durand, said a medical examiner has been brought to the base to determine the cause of death and an investigation will be conducted by the Naval Criminal Investigative Service, which is standard in the death of detainees at Guantanamo.

Durand said the U.S. government was working to notify the man's family and his country before releasing further information.

"Certainly we don't want the family finding out in the media before they have been notified," he said.

A mortuary team will wash and place the body in a shroud in accordance with Islamic burial rites before it is shipped back to the prisoner's homeland, he said.

The most recent death was in April 2011, when a 37-year-old Afghan prison died in an apparent suicide. His lawyer told The Associated Press at the time that the man had a long-term mental illness and had tried to kill himself at least once before. Two Saudis and a Yemeni prisoner were found hanging in their cells in June 2006 in what the military determined was a coordinated suicide.