The 79-page report, released Friday, said Adnan Farhan Abdul Latif died after downing two dozen pills he had been prescribed for mental illness.
Latif apparently horded his daily meds until he had enough to cause a fatal overdose, said the report, obtained by The New York Times.
Latif, a Yemeni national, suffered from a traumatic brain injury sustained in a car accident during his youth and was diagnosed by U.S. doctors as being bipolar and antisocial. The report said he had grown increasingly agitated and belligerent toward Guantanamo guards in the weeks before his death.
The Times said the report cleared up questions about how Latif was able to stockpile his medications and how he was able to take them with him when he was moved to a new cell. The report concluded, however, prison staff had been lax in keeping an eye on Latif.
Latif was captured in Pakistan in 2001 and confessed under interrogation he planned to enlist in the Taliban.
His release was approved by a court in 2010 but the United States had by then banned the repatriation of Guantanamo captives to Yemen.