(Portland, OR)  --  An investigation is underway into the death of an inmate at Oregon State Penitentiary.  Jail staff say 65-year-old John McCartney was found unresponsive inside his cell Tuesday morning.  Authorities say they believe he died of natural causes.  McCartney was serving time for four counts of first-degree sodomy in Washington County.  He was scheduled for potential release in early April.