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(Warning: This story contains extremely graphic and disturbing details) 

One convicted killer has been accused of beheading another in what authorities call an exceptionally sadistic torture slaying at a California prison.

Corcoran State Prison inmate Jaime Osuna removed several body parts from his cellmate, Luis Romero, Assistant Kings County District Attorney Phil Esbenshade said Friday. Charges accuse Osuna, 31, of repeatedly cutting Romero last month using what the prosecutor called a sharp metal object wrapped in string and attached to a handle.

It’s not clear how much happened while Romero, 44, was still alive or whether anyone heard the overnight assault, but “we do believe that the victim was conscious during at least a portion of the time,” Esbenshade said in an email. “This is the most gruesome case that I have seen in terms of heinousness in the slaying.”

The California Department of Corrections and Rehabilitation is conducting an internal investigation, spokeswoman Terry Thornton said. Officials wouldn’t provide more details on how prisoners are overseen overnight.

Osuna pleaded not guilty to murder and other charges at his first court appearance Thursday. They include several special circumstances that could bring the death penalty, including that the slaying “was especially heinous, atrocious, or cruel, manifesting exceptional depravity.”

Defense attorney Melina Benninghoff was appointed to represent him but was home sick Friday and did not respond to telephone and email requests to comment on his behalf.

Osuna also is charged with torture, mayhem and weapons possession. The torture charge alleges that he acted “with the intent to cause cruel and extreme pain and suffering for the purpose of revenge, extortion, persuasion and for a sadistic purpose.”

The state corrections department said guards found Romero dead in his cell about 7:30 a.m. March 9 at the prison, which houses more than 3,300 inmates about 220 miles (354 kilometers) south of Sacramento.

Romero bled to death from “multiple sharp force trauma injuries,” and his body was mutilated, according to an autopsy report released Friday.

Osuna was sentenced to life without parole after pleading guilty in 2017 to killing Yvette Pena, 37, at a Bakersfield motel in 2011, according to media reports at the time.

Romero also was serving a life term for a Los Angeles County slaying, but with the possibility of parole.

Osuna has been transferred to a Stockton prison for inmates needing medical or mental health care, though officials wouldn’t say why, citing privacy laws.