According to the Los Angeles Times, the California Corrections Department reported a stabbing in the facility on Saturday.
While the victim's name was not explicitly stated in the report, a government source confirmed to the Associated Press that 75-year-old Sirhan was stabbed by a fellow inmate on Friday at 2:21 p.m. local time.
Cal Fire spokesman Cpt. Thomas Shoots said that medics responded to a reported stabbing with the victim bleeding from the neck just before 2:25 p.m. Friday.
"Officers responded quickly, and found an inmate with stab wound injuries," the department told the Times. "He was transported to an outside hospital for medical care, and is currently in stable condition."
The department said that "the suspect in the attack has been identified, and has [been] placed in the prison's Administrative Segregation Unit, pending an investigation."
Sirhan was convicted of fatally shooting Kennedy on June 5, 1968, and sentenced to death, but the sentence was changed to life in prison after the death penalty was banned in California.
Kennedy was the brother of former US president John F. Kennedy, who was also assassinated – at the Ambassador Hotel in Los Angeles immediately after he declared his victory in the California Democratic primary.
The assassin said that he opened fire at the senator because he was infuriated by his support of Israel.
Sirhan's next hearing is slated for February 2021.