A San Francisco parolee was arrested Monday afternoon on suspicion of robbing a 15-year-old boy at gunpoint in Marin City earlier that day, the Marin County Sheriff’s Office said.The teen was robbed and beaten by a man in his mid-20s around 7:50 a.m. Monday in the 200 block of Drake Avenue. He was treated for his injuries at Kaiser Hospital and released, Barry said. Sheriff’s Sgt. Debra Barry said detectives believe the robbery may be connected to a shooting Sunday night in which four shots were fired at a car leaving a parking lot in that same block.The shooting, in which a bullet grazed the car but did not injure anyone, followed a failed attempt by the car’s occupants to buy marijuana, according to the sheriff’s office.Barry said investigators believe “some of the same persons are involved in both crimes.”Detectives identified the suspect in the teen’s robbery as Deandre Williams, 25, a parolee with a San Francisco address.Sheriff’s detectives arrested Williams around 4:15 p.m. Monday as he walked down Fillmore Street in San Francisco.Williams was booked into Marin County jail for making criminal threats, assault with a deadly weapon causing great bodily injury, battery with serious bodily injury, robbery, obstructing a police officer and a parole violation. His bail is $50,000 and he is scheduled to be arraigned this afternoon in Marin County Superior Court.