can I sue my county for negligent, for safty issue

It is possible that you have a cause of action for negligent security if there was some pattern of conduct in the past in the park that would suggest that a bigger police presence was needed to make the area safe. These are very fact sensitive cases and they are difficult to prosecute, but I have successfully prosecuted a case against a school district involving similar allegations. Click here for a discussion of that case. Click here and here for more information about me.