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.

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

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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.

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