Officers say they heard a loud noise during security checks just before 9 p.m. Friday night.
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When they followed the noise it led them to a cell, where 24-year-old Johnny Lopez was lying unresponsive on the floor.
His cellmate, 34-year-old Eduardo Gutierrez, stood above him as prison staff ran in to try to save Lopez.
About an hour later, Lopez died.
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The prison's Investigative Services Unit has launched an investigation into the incident.
Lopez was booked in August 2017 with a three-year sentence for first-degree burglary out of Ventura County.
Gutierrez was transferred in May 2013 from San Diego County with a 24-year sentence for voluntary manslaughter, personal use of a deadly weapon, and possession/manufacture of a deadly weapon as a second-striker.