A Yemeni detainee at Guantanamo Bay has died in his cell, U.S. military officials said Tuesday. However, investigations have been ongoing to determine how the so-called Mohammed Ahmed Abdullah Saleh Al-Hanashi died.

During a routine visit inside the cells on Monday nights, the guardians at Guantanamo Bay have found Al-Hanashi unconscious, the sources said, adding that after intensive procedures have been taken, doctors announced the detainee's death.

Al-Hanashi had been held on occasion in the detention center psychiatric ward, although detention center officials would say neither whether he was found dead in a cell there nor how he had died.

Al-Hanashi was captured at Mazar e Sharif during the U.S. invasion of Afghanistan in late 2001. His detainee documents say he went to Afghanistan before the Sept. 11, 2001 attacks ''to participate in jihad'' with members of the Taliban.

Human rights groups blamed the U.S. indefinite detention regime that has evolved in the war on terror."Tragic deaths like this one have become all too common in a system that locks up detainees indefinitely without charge or trial,'' said Ben Wizner, staff attorney at the American Civil Liberties Union.

He urged an immediate, independent and transparent investigation into the circumstances surrounding this apparent suicide and the conditions of confinement at Guantánamo.''

Four detainees who killed themselves in 2006 and 2007 were found hanging. Three were Saudis. One was Yemeni. If confirmed, Al-Hanashi will be the sixth death case overall and the fifth suicide case that the U.S military at Guantanamo detention announced since it has been established in January 2002.