Terrence Barnett has been found guilty of killing a Polk County sheriff's deputy at the South County Jail in 2009.
A jury Friday convicted Barnett of murder in the death of Sgt. Ronnie Brown.
Barnett was already serving a 30-year sentence at the jail for his involvement in the 2007 death of a man in Highlands County.
Prosecutors said Brown got into a struggle with Barnett after the inmate became agitated and broke a sprinkler head in his jail cell. Barnett then shoved Brown backward against a wall, causing him to fall onto his back.
Brown died days later at Winter Haven Hospital after complications from back surgery.
"We appreciate the jury's service to Polk County. We know serving on a jury is a difficult and important task," Polk County Sheriff Grady Judd said Friday in response to Barnett's conviction. "We continue to pray for Ronnie's wife, children, and grandchildren. Hopefully this conviction will help bring some closure for their family and our PCSO family."
Prosecutors said they planned to pursue the death penalty against Barnett.