A guard on duty in the yard of a central California prison was shot by someone outside the facility, and a suspect is still at large, authorities said Tuesday.

Between 15 and 20 rounds were fired from a high-caliber weapon Monday evening at North Kern State Prison in Delano, a rural area about 35 miles north of Bakersfield.

The officer, who was struck once in the torso, was treated at a hospital for minor injuries and released.

"The round penetrated his stab-resistant vest, but the wounds were minor. It just scraped his side," said California Department of Corrections and Rehabilitation spokeswoman Terry Thornton.

The officer's name was not released. It was not clear if the shots were intentionally fired at the officer, or if he was hit by bullets fired generally at the prison, officials said.

Several law enforcement agencies, including a sheriff's helicopter crew, joined the search for a suspect or suspects. The active search ended early Tuesday but the investigation was ongoing, said Kern County sheriff's spokesman Ray Pruitt.

No weapons were recovered, but ammunition casings were found.

Thornton said inmates were locked in their cells, except for those who had medical appointments or work in the kitchen.

Officials said they do not recall ever before having had a correctional officer shot from outside a prison, though guards have been shot inside prisons during escape attempts.

Department officials planned a mid-day news conference at the institution. North Kern State Prison opened in 1993 and houses approximately 4,300 inmates.

___

Associated Press writer Don Thompson contributed from Sacramento.