An autopsy shows that the 17-year-old Wisconsin girl who died at a music festival near Detroit Lakes this summer died of a drug overdose.

Becker County Sheriff Tim Gordon released the results of the final autopsy report on Ashley Marie McCoy of West Salem, Wisconsin, on Tuesday.

Gordon says McCoy died accidentally from an overdose of the prescription painkiller Oxycodone, which was prescribed for her after a recent surgery.

He says the autopsy also found methadone in her system. The prescription drug is best know for curbing heroin addiction, but is also prescribed as a painkiller.

Previously, authorities said McCoy was found dead at her campsite at the 10,000 Lakes festival in June. She died in her sleep.