CHINO VALLEY (AP) — A wildfire sparked on the grounds of a state prison in Southern California, but corrections officials say it is not a threat to inmates or the community.

The fire started after noon Friday in a field next to the California Institution for Men in Chino, 36 miles east of Los Angeles. It had burned about 30 acres on the 2,500-acre prison grounds by late afternoon.

Firefighters from four agencies were attacking the blaze, which was about 60 percent contained by late Friday evening.

There were no reported injuries or damage to property. No evacuations have been made. The prison’s nearly 5,000 inmates were locked in their cells as a precaution.

Officials say they preliminarily believe the fire was started by a mechanical malfunction in a prison vehicle being driven near dry vegetation.