MIAMI (Reuters) - A Yemeni captive died in an apparent suicide at the detention center for foreign terrorism suspects at the Guantanamo Bay U.S. naval base in Cuba, the U.S. military said on Tuesday.

Detainees talk together inside the open-air yard at the Camp 4 detention facility at Guantanamo Bay U.S. Naval Base in Cuba, May 31, 2009. REUTERS/Brennan Linsley/Pool

It was the first death among Guantanamo prisoners since U.S. President Barack Obama took office in January and ordered the camp shut down by January 2010. His effort to empty the prison has been beset by frightened U.S. politicians who do not want Guantanamo prisoners moved to maximum-security prisons in their constituencies.

A military statement said 31-year-old Muhammad Ahmad Abdallah Salih, also known as Al Hanashi, “died of an apparent suicide” on Monday night, but did not say specifically how he died.

Human rights groups condemned the death and said it underlined the need to end the system of “indefinite detention” at the prison camp that opened in 2002 under the Bush administration to hold terrorism suspects after the September 11 attacks on the United States that killed 3,000 people.

The latest death was the sixth among the camp’s captives. Four had committed suicide and one had died of natural causes.

Guantanamo has been a focus of international criticism for denying detainees legal rights. The Obama administration is considering what to do with the 239 remaining captives held at Guantanamo, who include nearly 100 Yemenis.

Guards found the prisoner unresponsive and not breathing when they checked his cell on Monday night, according to the U.S. military’s Southern Command, which has responsibility for the Guantanamo base.

“Medical personnel were immediately summoned by the guard force. After extensive lifesaving measures had been exhausted, the detainee was pronounced dead by a physician,” the Southern Command said in a statement.

The Naval Criminal Investigative Service has begun an investigation to determine how he died, the U.S. military said.

HUNGER STRIKES

The dead man had been held without charge at Guantanamo since February 2002. He had been on hunger strikes in the past to protest his detention, but was not among the more than two dozen long-term hunger strikers currently being force-fed at the camp, a Guantanamo spokesman said.

His most recent hunger strike ended in mid-May, said the spokesman, Navy Lieutenant Commander Brook DeWalt.

An autopsy was pending and a “cultural adviser” was ensuring that his body was being treated respectfully and in accordance with Muslim tradition, the U.S. military said.

“Upon completion of the autopsy, the remains will be prepared for repatriation to Yemen,” the statement added.

Human Rights First condemned the death as “a stark reminder of the inhumanity of indefinite detention without charges or trial.”

The American Civil Liberties Union said it illustrated the need to resolve the detainees’ fate in a regularly constituted court with long-established rules.

“There is no room for a system of indefinite detention without charge or trial under our Constitution,” the ACLU said. “Those against whom there is no legitimate evidence must not be given a de facto life sentence by being locked up forever.”

Since the detention camp opened in January 2002, four prisoners have hanged themselves in their cells -- three on the same day in June 2006 and one in May 2007. Another prisoner died of colorectal cancer in December 2007.

More than 530 Guantanamo prisoners were freed or transferred to other countries under the Bush administration, and two have been released since Obama took office in January.

The Obama administration is still sorting the remainder into four groups -- those to be released or sent to other countries, those to be tried in the regular U.S. courts, those to be tried in revised military tribunals and those to be held indefinitely, because they cannot be prosecuted but are judged to pose a threat.