BOSTON — Two people were walking down a Dorchester street Wednesday evening when three individuals came up behind them, punched one of them in the back of the head, grabbed his phone, and took off running.

The victim who was struck went to the hospital. The phone was gone. The suspects had vanished toward Dorchester Avenue.

That was at 7:07 p.m.

By 7:42 p.m., Boston police had one of them in handcuffs. He is 14 years old.

Officers from District C-11 responded to the area of 42 Charles Street after the victims reported being attacked while walking along Park Street. The assault came without warning. Three suspects, from behind, one punch to the back of the head, one phone snatched, then all three running.

Boston EMS transported one victim to a local hospital with non-life-threatening injuries.

What happened next was fast. Detectives circulated a photo of potential suspects. Within 35 minutes of the original call, officers spotted two individuals nearby who matched the description. One of them, a 14-year-old, was placed under arrest. The other was released.

Police have not said whether the other suspects have been identified.