(CNN) The gunman who killed two doctors and two others at an Oklahoma medical building Wednesday did so after he blamed one of the physicians for causing him pain from a recent back surgery, a police chief said Thursday. The shooter, Michael Louis, who police said killed himself after the rampage in Tulsa, had just earlier that afternoon legally purchased one of the firearms used in the slaughter, an AR-15 style rifle, city Police Chief Wendell Franklin said during a news conference.