If he was hundreds of yards away this was almost certainly a professional hit attempt.
Not “almost certainly.” Many people could shoot that well, even with conventional weapons. But. In fact, many, many rifles are now made and sold that are capable of killing at 1,000 yards. The real question is: “Why was a rooftop within (450 to 200 yards, depending on the reports) such close range left unoccupied by cops?