MARTINEZ (CBS SF) — A Walnut Creek police officer has been charged for allegedly donning a mask and beating a woman with a baseball bat in Richmond last month, a Contra Costa County prosecutor said Monday.

Gregory Thompson, a 54-year-old Martinez resident and 30-year Walnut Creek police veteran, was charged this morning with felony assault likely to cause great bodily injury, felony vandalism and being armed in the commission of a felony, Deputy District Attorney Barry Grove said.

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Thompson’s arraignment on the charges is scheduled in Contra Costa County Superior Court Monday afternoon.

Richmond police said officers arrested Thompson in the early morning hours of Aug. 16 after several residents called around 2 a.m. to report a woman screaming and a man with a bat in the 4000 block of Clinton Avenue in Richmond.

As officers arrived on the scene, witnesses pointed out Thompson, who was sitting nearby in his parked car. He wasn’t wearing his police uniform or in a patrol car at the time, police said.

Officers approached the suspect and noticed a mask lying on the car’s floorboard. A search of the car turned up a baseball bat, two guns and zip ties, police said.

Police said the victim was taken to a hospital for injuries suffered in the beating and was expected to recover.

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Officers learned the woman had been walking in the area looking for assistance after her car ran out of gas when she was attacked.

Police last month were also attempting to locate a second woman who was also apparently wounded when she stepped in to try to stop the attack.

Thompson, who identified himself as a police officer, was arrested and taken to county jail in Richmond and released on bail the following day.

He has remained out on bail but is expected to surrender at the county jail in Martinez this afternoon, Grove said.

Walnut Creek police have said Thompson was placed on administrative leave soon after his arrest and that the department is conducting its own investigation.

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He was working most recently as a patrol officer, according to Walnut Creek police Capt. Mark Perlite.