UPDATED: 18 Salinas Valley State Prison Inmates Injured in Riot

Eighteen Salinas Valley State Prison inmates suffered stab wounds or head trauma after a riot broke out Tuesday morning on the prison yard.

A preliminary prison investigation showed 69 inmates were involved in the riot, which left one severely injured man airlifted to a hospital, 10 others hospital-bound by ambulance and seven others in care at the prison's on-site medical facility.

One prison staffer was hurt while responding, though the injuries were not intentionally inflicted by an inmate, according to a statement released Tuesday.

Prison officials are investigating the cause of the riot, which broke out at about 11:10 Tuesday morning. Correctional officers used pepper spray to quell the violence, according to prison officials.

At least five ambulances responded to the scene. Prison staff did not call on outside law enforcement to help contain the incident, but did receive outside medical response assistance. An American Medical Response supervisor is working with prison staff to assess all inmates who were on the yard at the time of the riot to catch any injuries that might have gone unnoticed earlier, according to prison officials.

The minimum- and maximum-security prison opened in 1996 in Soledad, and will operate on a modified program until a review of the riot is completed.

Note: An earlier version of this story stated 159 inmates had been involved in the riot. Prison officials adjusted the estimated number to 69 after an initial investigation.