The question is whether the police officer knew the dog was in the fenced yard before he entered. If he did then most likely he knew the suspect was not in the yard. In this case I believe without evidence that the teen was in the yard he should have waited for more officers to search. The yard could have been cordoned off, and the owner could have been contacted to control the dog. The department should pay the owner for his loss. The officer should use more common sense next time, if he had missed he likely would have been mauled.