Adnan Latif, who was found unconscious in his cell in a disciplinary wing of the prison at the U.S. base in Cuba, took an overdose of psychiatric medication, according to a senior Defense Department official.
The official said it had not yet been determined how Latif, who was from Yemen and had a history of mental illness and clashes with guards, managed to collect enough medication for a fatal dose. Investigators are looking into whether he hoarded pills to take all at once.
The official spoke on condition of anonymity because the autopsy results have not yet been released and the case remains the subject of an investigation by the Naval Criminal Investigative Service.
The U.S. military does not intend to disclose the results of the autopsy or discuss the case in further detail until after Latif's remains are returned to his country, said Army Col. Greg Julian, a spokesman for the U.S. military's Miami-based Southern Command, which has jurisdiction over Guantanamo. The remains are at the Ramstein Air Base in Germany.
His death was the seventh suicide at the prison, where the U.S. now holds 166 men. The deaths of two other prisoners were determined to be from natural causes.
The finding of suicide in the Latif case was first reported by the website Truthout, followed by The New York Times.
David Remes, a lawyer for Latif, said he is skeptical of the military's conclusion and said it would be difficult for the prisoner to accumulate enough medication to commit suicide because he was frequently searched and monitored inside the prison.
Remes said he thinks it's possible that Latif was given excessive or incorrect medication. "Given a choice between blaming themselves and blaming Adnan, the choice they made was all but preordained," he said.
Latif, who was in his 30s, was held at Guantanamo for more than a decade. The U.S. government accused him of training with the Taliban to fight the Northern Alliance in Afghanistan. He was never charged and was apparently among about 30 prisoners from Yemen who could not be sent back because their country is considered too unstable to prevent former prisoners from engaging in militant activities.
At Guantanamo, he was among dozens of prisoners who waged hunger strikes to protest their captivity. At the time of his death he was in a disciplinary unit for allegedly hurling bodily fluids at a guard.
He had challenged his confinement with a civil petition known as a writ of habeas corpus. In July 2010, a judge ruled a classified report was insufficient evidence that he had trained at the militant camp and ordered his release.
The government appealed to a higher court, which ruled that courts should assume the government documents were accurate and reliable. In June, the Supreme Court declined to hear his appeal, which lawyers said has caused increased despair among Guantanamo prisoners.
Associated Press writer Pauline Jelinek in Washington contributed to this report.