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Bay Area News Group via AP, Laura A. Oda
Law enforcement members gather at Eden Medical Center in Castro Valley, Calif., on Wednesday, July 22, 2015 where Hayward Police Sgt. Scott Lunger was transported after being shot and killed during a traffic stop. Lunger, 48, was shot in the early morning after he stopped a white Chevrolet pickup that was swerving on the road and driving erratically. Authorities say the shooter hasn't been arrested.

SAN FRANCISCO — Hayward's police chief says a 21-year-old has been arrested in the shooting death of a police sergeant killed during a traffic stop.

Chief Diane Urban says Mark Estrada of Oakland is being treated for gunshot wounds and under police guard at an Oakland hospital.

Urban says Estrada has gang ties and that he was taken to Highland Hospital in Oakland by relatives.

It's unclear whether Estrada is represented by a lawyer who could comment on the arrest.

Sgt. Scott Lunger was shot early Wednesday after he stopped a white Chevrolet pickup that was swerving on the road and driving erratically.