Initial reports indicated that Matthew, 17, was surrounded by as many as four people before one of them shot him in the head.
The Suffolk County District Attorney's office says Matthew was specifically targeted by the group:
Cell tower records, video surveillance footage, and other evidence suggest that Clark was called to the area, where he met and joined with others to find Matthew. Galatis said he shot Matthew once in the head before fleeing and was identified out of a number of suspects as the one who fled the scene of the shooting with a gun in his hand.
Clark was ordered held without bail at his arraignment today in Roxbury District Court.
Police and the DA's office say the investigation continues and that additional arrests are possible.