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CHILLICOTHE, Ohio (AP/KDKA) – A prosecutor says investigators haven’t been able to determine how an inmate and 23 staff members at a southern Ohio prison were exposed to and sickened by a mix of heroin and the potent painkiller fentanyl.

Ross County Prosecutor Matt Schmidt tells the Chillicothe Gazette that authorities couldn’t determine who brought the contraband drugs to Ross Correctional Institution and don’t have enough information to prosecute anyone for the August incident.

It was investigated by the State Highway Patrol, which hasn’t publicly released its report.

Schmidt says the investigation was inconclusive about what caused the exposure, but one possible explanation came from the inmate who was treated, who told investigators the fan in his cell blew dust or white powder into the air when it was turned on that day.

Dozens of prison employees at facilities across Pennsylvania were sickened in August, prompting a lockdown of all 25 Pennsylvania state prisons. Officials believed the employees were likely exposed to K2, a synthetic liquid form of marijuana that can be sprayed on letters or book pages.

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