A detainee has died of undetermined causes this weekend at the U.S. military detention center at Guantanamo Bay, the U.S. military says.

The prisoner -- whose name and nationality had not been released -- was the ninth detainee to die at the base since it opened in January 2002, the Associated Press reports.

The U.S. Southern Command says in a statement that a medical examiner was brought in to determine the cause of death.

Suicide has not been ruled out, according to Southern Command Sgt. Shanda De Ande, CNN reports.

The detainee was found in his cell unconscious and unresponsive by guards on Saturday during a routine check and taken to a base hospital where he was declared dead "after extensive lifesaving measures had been performed," the U.S. military says.