Arkansas authorities are conducting an investigation after an inmate at a maximum security prison was found dead in his cell.

The Arkansas Department of Correction says staff at the Maximum Security Unit in Tucker found 49-year-old David Mulkey unresponsive in his single-person cell on Sunday afternoon. Mulkey was taken to the infirmary where he was pronounced dead.

Arkansas State Police was immediately notified and is conducting an investigation into the death. The Correction Department says it’s also conducting an internal investigation.

Mulkey was serving a life prison sentence for a first-degree murder conviction out of Pulaski County.