The South Dakota Department of Corrections said James Strahl, 46, passed away at a Sioux Falls hospital July 14 following an extended illness.
Strahl had been sentenced to 25-years for 1st degree manslaughter in the death of William O'Hare in 1998.
Strahl was actually convicted of murder in 2007.
But, Strahl got a new trial after testimony from his cellmate was thrown out.
Rather than face another trial, Strahl took a plea bargain.
He was also serving a consecutive 10-year sentence for grand theft.
Officials will conduct an autopsy.
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