SAN DIEGO (CNS) – Two prison inmates from San Diego County were under investigation Friday for allegedly stabbing a fellow inmate to death at the Central California penitentiary where they are serving lengthy terms for violent crimes.

Michael Arzaga, 47, and Matthew Thornton, 38, allegedly attacked 48- year-old convicted murderer Robert Hargrave at Kern Valley State Prison in Delano with makeshift weapons about 1:30 p.m. Thursday, the California Department of Corrections and Rehabilitation reported.

Guards used chemical agents to halt the assault, according to CDCR public affairs. Medics took Hargrave, who had suffered puncture wounds to his back, chest and abdomen, to a prison medical center. He was later transferred him to an outside hospital, where he was pronounced dead.

Hargrave had been incarcerated since May 1994, after being sentenced in Riverside County to life without the possibility of parole for first-degree murder.

Arzaga was convicted of second-degree murder in 1998 and was serving a life sentence with the possibility of parole. According to the San Diego Union-Tribune, Arzaga was convicted in October 1997 for stabbing his girlfriend to death. She was found dead in a Midway District motel room.

Thornton was admitted to the penitentiary north of Bakersfield in 2009 to serve a 19-year sentence for attempted murder. He was convicted for stabbing a man outside Bub’s Dive Bar on Garnet Avenue in March 2007.

A prison investigative unit and the Kern County Sheriff’s Office are investigating the incident.