From an article:
The incident started at 6:44 p.m. when, according to witnesses, a male got out of a vehicle stopped in traffic near 85 Southampton St., walked up to a car that was also stopped in traffic about two or three cars ahead, and fired into that vehicle.
The gunman then returned to the car and got in the back seat. According to Suffolk prosecutor Jennifer Hickman, Hill was driving and Johnson was in the front passenger’s seat.
A Boston police detective was nearby and heard the gunfire. The detective, in his cruiser, pursued the suspects’ vehicle, and had managed to trap it in traffic momentarily, before cars ahead moved and the suspects’ vehicle took off.
A state trooper then started pursuing the vehicle and stopped it on the Southeast Expressway near the Columbia Road off-ramp. The three suspects were taken into custody at gunpoint.
The article didn't say where the boy got the gun.
Every gun in the hands of a child must first pass through the hands of an adult.