CONCORD (BCN) - A 17-year-old boy was robbed Wednesday afternoon while he was walking down a street in Concord, Concord police Cpl. James Carroll said Thursday.

The boy was walking on Monument Boulevard near Oak Grove Road at about 2 p.m. when a car pulled up alongside him and two teens got out.

The suspects asked the victim about gang affiliation and stole his backpack, which had $100 in it, Carroll said.

An off-duty federal immigration agent was in the area and detained one of the suspects, a 17-year-old boy. The second boy, also 17, got away, Carroll said.

The name of the suspect in custody is being withheld because he is a juvenile.

The victim was not hurt, Carroll said.

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