(CNN) -- A prison guard was killed and several employees injured Sunday in a riot at the Adams County Correctional Facility in Natchez, Mississippi, officials said.
The 23-year-old guard appeared to suffer "blunt trauma to the head," said Adams County Coroner James Lee.
The riot, which began about 2:40 p.m., was still going on Sunday night, the facility's operator said in a statement. Local and state law enforcement officials as well as authorities from the Federal Bureau of Prisons were helping the facility quell the violence.
"The disturbance is contained within the secure perimeter of the facility, with no threat to public safety," the statement said.
Five employees and one inmate were taken to a hospital for treatment of unspecified injuries, while additional staff members were being treated at the prison.
Johar Lashin told CNN that he'd heard a lot of noise and commotion when he talked around 6 p.m. with his brother Jawad, an inmate at the Natchez facility serving time for aiding and abetting illegal immigrants. His brother said he was not participating in the riot, despite pressure from other inmates to do so.
The cause of the incident is under investigation.
Rusty Boyd, a spokesman with the Mississippi Highway Patrol, said Sunday evening that 45 to 55 units from that state agency are helping corrections officers deal with the situation.
The facility is a 2,567-bed prison that houses adult men who are in the United States illegally and charged with crimes. It is owned by the Tennessee-based Corrections Corporation of America.
Warden Vance Laughlin described the facility as quiet and with "few problems" in a March 2010 article in The Natchez Democrat, a few months after it opened to incarcerate illegal immigrants detained for mostly low-security crimes. At that point, it contained more than 2,000 inmates -- more than two-thirds of whom were of Mexican descent, although scores of nationalities were then represented.
CNN's Darrell Calhoun contributed to this report.