“Falsely representing” yourself as a police officer is a crime. Possessing a uniform and mock police car is not a crime according to statute.
Even if the police had known that Wortman had a uniform and a mock police car, there was no basis to arrest him until he was actually pretending to be a policeman. Changing this law might have prevented the shooting, especially if it is clear that others were involve in getting the uniform and/or creating a mock police car.