Guards at the U.S. Naval Base in southeast Cuba found the detainee in his cell unresponsive and not breathing Wednesday afternoon, the U.S. military's Southern Command said in a statement.
"They tried to save his life but he was pronounced dead," said Mario Alvarez, a Miami-based spokesman for the command.
If suicide is confirmed, it would be the fourth such death reported at Guantanamo since the prison camp opened in January 2002. On June 10, 2006, two Saudi detainees and one Yemeni hanged themselves with sheets. Details, including the prisoner's name and manner of death, were not released.
A spokesman for detention operations, Navy Cmdr. Rick Haupt, declined to comment, referring questions to the Miami-based Southern Command, which oversees U.S. military operations in the Caribbean and Latin America.
The Southern Command subsequently issued a statement pledging that additional information will be provided as details become available.
"The Naval Criminal Investigative Service has initiated an investigation of the incident to determine the circumstances surrounding the death," the statement reads. "The mission of detention and interrogation at Guantanamo continues. This mission is vital to the security of our nation and our allies and is being carried out professionally and humanely by the men and women of Joint Task Force Guantanamo."
The U.S. military, in its statement, also noted that the "remains of the deceased detainee are being treated with the utmost respect. A cultural advisor is assisting the Joint Task Force to ensure that the remains are handled in a culturally sensitive and religiously appropriate manner."
The death came as the U.S. military prepares to try two detainees — Salim Ahmed Hamdan, a Yemeni, and Omar Khadr, a Canadian who was 15 when he was captured in a firefight with U.S. troops in Afghanistan. Their arraignment is scheduled to proceed on Monday at Guantanamo as planned, Navy Cmdr. Jeffrey Gordon, a Pentagon spokesman, said late Wednesday.
Wednesday, Khadr fired his American attorneys, leaving him without defense counsel, according to his former U.S. military attorney. Khadr is one of only three of the roughly 380 Guantanamo prisoners to be charged with a crime.
The military toughened security at the prison camp following the previous suicides and an uprising last spring, taking measures to remove access to light fixtures and other possible makeshift weapons.
Detainees at the isolated prison camp - some captured years ago - are being held by the U.S. military on suspicion of links to al Qaeda or the Taliban.
The Naval Criminal Investigative Service is conducting an ongoing investigation into three previous suicides reported at Guantanamo Bay.