Three suspects have been arrested and a fourth is being sought in the brutal attack of an 18-year-old man on a crowded San Francisco Municipal Railway bus last week, police Sgt. Lyn Tomioka said tonight.

Christopher Borgzinner, a local actor, was on a 9-San Bruno bus at about 5:45 p.m. Oct. 5 when the suspects approached him and asked if they could use his phone. Borgzinner complied and the suspects then asked him whether he belonged to a gang before assaulting him and stealing his iPod and wallet, Tomioka said.

Borgzinner, who has no gang affiliation, suffered a fractured orbital socket and other injuries, Tomioka said. He has since been released from the hospital.

Tomioka said police arrested a juvenile in the city earlier this week. Uluao Mase, 18, of San Francisco, was arrested in the Bayview District on Thursday, and a third juvenile was arrested in the East Bay today.

Tomioka credited the role of surveillance footage taken from Muni in making the arrests. “It was paramount in the identification and subsequent arrests of the suspects,” she said.

The two juveniles were arrested for assault, robbery and conspiracy, Tomioka said. Mase, who was allegedly in possession of a MAC-10 when he was detained, was arrested for robbery, assault, conspiracy, possession of a concealed weapon and possession of an assault weapon.

Investigators are still working to locate the fourth suspect, a juvenile. Anyone with information regarding the case is asked to call police at (415) 575-4444.

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