Greenville County Coroner Parks Evans told local media that 45-year-old Bennie Ray Brown Jr. of Pelzer died Friday at the Perry Correctional Institution.
The coroner said medical workers tried to revive Brown after he went into cardiac arrest but were unsuccessful.
Brown was convicted in 2016 of shooting and killing Nicole Kingsborough and Laurens County Sheriff's Deputy Roger Rice in 2011.
Police say Brown had followed Kingsborough to work and shot her in the head outside a plant in Fountain Inn. Prosecutors say Brown killed her because she had ended a nearly 20-year relationship with him.
Rice was killed when he and other officers went to question Brown about Kingsborough's death.
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