We believe that our family dog got poisoned and she has died. What do I need to do to pursue criminal charges?

On Thanksgiving day we noticed that our dog was sick. She couldn't hardly walk and was acting very strange. We believe that she was poisoned. There is a man that lives very close to us that has been accused of poisoning many animals over the years. When we moved in the neighbors even told us to " put the dog pen far away from the property line" so that he couldn't throw poison into the pen. We have no physical proof that he did anything wrong and our dog has been missing for about 3 days so we can't take her to the vet and have an "autopsy" done. What can we do to pursue criminal charges or at least try to keep this from happening again? I feel like I am helpless and he has gotten away with this for way to long and I don't want to sit around and let it happen again?