The death of an inmate at a Tucson prison over the summer has been ruled a drug overdose.

Carlo Krakoff, 36, died from an accidental heroin overdose on July 31, said a news release from the Arizona Department of Corrections.

Krakoff was found unresponsive in his cell at the Arizona State Prison Complex-Santa Rita Unit, the release said. Krakoff was serving a 13-year sentence for armed robbery.

Prison officials have not said how Krakoff was able to obtain heroin.