Cops can only do so much when people refuse to talk or cooperate with investigations. It's a waste of time. Don't call the cops for help and then refuse to talk. People know who the usual suspects are in violent crimes. Crimes aren't committed in a vacuum, people talk. So when police ask questions, be truthful, name names, mention reasons and possible motives. Otherwise you are wasting valuable resources that could be invested helping people that actually want help.