Officials believe that Romero, who was found dead March 9, bled to death after Osuna attacked him with a "sharp metal object wrapped in string and attached to a handle," the AP reported. Romero suffered "multiple sharp force trauma injuries" in the overnight attack, an autopsy found.
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Prosecutors said Osuna decapitated Romero and removed other parts of his body, KFSN-TV reported.
"We do believe that the victim was conscious during at least a portion of the time," Kings County Assistant District Attorney Phil Esbenshade told the AP in an email.
Romero, who was convicted of second-degree murder in 1992, had been serving a sentence of life with the possibility of parole, the Bakersfield Californian reported.
On Thursday, Osuna pleaded not guilty after he was arraigned on multiple charges, including murder, torture, mayhem and weapons possession, the AP reported. If convicted, he could face the death penalty.
– The Associated Press contributed to this report.