Walla Walla County Coroner Richard Greenwood ruled the death of 47-year-old David Brewczynski on Aug. 31 a suicide.
Walla Walla police and the Department of Corrections are investigating.
Penitentiary spokeswoman Shari Hall says she can't comment while the prison is conducting a critical incident review.
KREM reports Brewczynski was convicted of murder and theft in the 2008 killing of 80-year-old Kenneth Cross during a burglary at his Spokane Valley home.
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