California Department of Corrections and Rehabilitation (CDCR) officials are investigating after a death row inmate was killed at Kern Valley State Prison in Delano on Friday.

Convicted murderer Mario Renteria, 36, allegedly started beating fellow inmate, Julian Mendez, 46, at about 10:30 a.m. Friday, prompting prison staff to respond. 

Officers ordered them to get down, but the men failed to comply, according to a CDCR news release obtained by Fox News Digital.

Chemical agents initially stopped the attack, but more than 30 additional inmates rushed Renteria and began striking him.

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Orders to stop were ignored, and staff used multiple blast grenades to quell the violence, according to CDCR.

Mendez suffered multiple wounds, and life-saving measures were immediately taken. He was taken to the prison's triage and treatment area, where a doctor pronounced him dead at 11:05 a.m.

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Officials said an improvised weapon was found at the scene, though the type of weapon was unclear.

Renteria remains in restricted housing pending investigation, according to CDCR. 

Officials limited population movement to facilitate the investigation by the prison's Investigative Services Unit and the Kern County District Attorney’s Office. 

The Office of the Inspector General was notified, and the Kern County Coroner will determine Mendez’s official cause of death.

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Mendez was received from Riverside County on Dec. 2, 2004, according to CDCR. He received a condemned sentence in 2002 for the first-degree murder of two teenagers.

CDCR said Renteria was received from Riverside County on April 27, 2022, and was sentenced to life with the possibility of parole for first-degree murder (a third-strike offense) and arson.

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Kern Valley State Prison opened in 2005 and houses over 3,100 minimum- and high-security-custody inmates.

A man's Facebook plea for a liver donor attracted the attention of a high-school acquaintance he hadn't spoken to in 20 years.

In April 2024, after experiencing appetite loss and losing 15 pounds in a month, Steven Register, 42, was diagnosed with stage 4 colon cancer, according to SWNS.

Doctors told the former MLB pitcher — who played for the Colorado Rockies in 2008 and the Philadelphia Phillies in 2009 — that a liver transplant was likely his best chance of survival.

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"When we first met with the doctors, they gave me a year to a year-and-a-half to live," Register said, adding that he and his wife, Beth, immediately started researching options.

The couple traveled from Auburn, Alabama, to the MD Anderson Cancer Center in Houston, Texas, where Register underwent surgery for a temporary ostomy bag and began chemotherapy, per the SWNS report.

Plans for the liver resection were canceled when doctors discovered the tumors were too large, which led the couple to consider a transplant.

Register's wife created a Facebook group to search for a living liver donor, hoping someone would come forward in time.

Kristin Johnston, a 40-year-old preschool teacher from Roswell, Georgia, saw the post and recognized Register as a former high-school classmate.

The two had met in 1999 at Shaw High School in Columbus, Georgia, but hadn’t spoken in over two decades, SWNS reported.

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"It was just so far out of left field," said Register, who couldn’t believe it when Johnston volunteered to donate part of her liver.

He added, "I haven't seen or talked to her in over 20 years, and for her to reach out like that, it was just meant to be."

Johnston said she started by doing a quick online search for live liver donation and discovered that blood type compatibility was the first step.

"I just sent him a message," she said. "I said 'hey, what's your blood type?' and he said, 'I'm B positive.'"

She responded, "Wait, that’s mine too," and offered, "I’ll happily donate a lobe if I’m a match."

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Johnston was later cleared as a match and received the confirmation on a meaningful day.

"I got the news on Good Friday, of all days, that I was officially cleared as his liver donor," she said.

The surgery, which is expected to take 12 to 14 hours, will involve removing 70% of Johnston’s liver and transplanting it into Register, according to SWNS.

Both her remaining liver and the donated portion will regenerate over time, giving both a second chance at health.

"For her, she is ultimately giving him the gift of life — for him, a really fresh start in this journey," said Beth Register.

Steven Register added, "Hopefully, once I get my liver with all the tumor and cancer out, I’ll be cancer-free from there."

The Register family — including children McKenzie (16), Blakely (14) and Brooks (8) — launched a fundraiser on SupportNow to help with travel, food and medical expenses.

Johnston and her husband Cody, 38, a real estate developer, have three kids of their own — Sawyer (9), Teddy (7) and Dahlia (4).

Beth Register said she hopes their family's story raises awareness about the impact of living organ donation.

"A lot of people don’t realize that live donations, whether it be for livers or kidneys, are even an option," she said.

She added that Johnston has been selfless from the start.

The families believe fate had a hand in reconnecting them, per SWNS.

"We just pray that God is opening all the right doors and that Kristin is the perfect donor for him," Beth Register said.

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She continued, "We just appreciate her being willing to put her life on pause to hopefully lengthen his life by many, many, many years."