San Francisco police today announced the arrest of the fourth and final suspect in connection with an October attack on an 18-year-old actor who was riding on a San Francisco Municipal Railway bus.

Roger Alarcon, 18, of San Francisco, was arrested by local police in Richmond and was in custody at San Francisco jail today, police said.

He was the only outstanding suspect wanted in connection with the assault on Christopher Borgzinner on a 9-San Bruno bus at 5:45 p.m. Oct. 5.

In October, police arrested Uluao Mase, 18, and two male juveniles, ages 16 and 17, all of San Francisco.

The attack left Borgzinner with a fractured orbital socket, cheekbone and jaw, according to prosecutors.

Police and prosecutors have said Mase confronted Borgzinner on the bus and asked to use his cell phone. Mase and the other suspects then allegedly asked if Borgzinner belonged to a gang before assaulting him and stealing his iPod and wallet.

Borgzinner, a San Francisco resident, has no gang affiliation, police said. He plays “Nacho” in the upcoming film “La Mission,” starring Benjamin Bratt and directed by Bratt’s brother Peter Bratt. The film was shot in San Francisco’s Mission District.

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