need the answer

a policeman is walking down an alley when he hears a scream from the window above,"John,John, don't Shoot!" he runs up to the room when he hears a gun shot. in the room there is a doctor, a lawyer, a man in a green suit and a lady dead on the ground with a gun next to her. after observing the scene he walks to the man in the green suit and says, "John, you're being arrested for murder."

How did the Policeman know?