To be honest that sounds very much like a fox to me. Some years ago I had the exact same thing happen and was absolutely convinced for ages that it was a person. We had the police out too, but eventually it transpired that in spite of having lived there for 7 years and never having seen a fox, we had suddenly acquired one! The muddy pawprints which appeared on the corrugated plastic top to the run a few weeks later were a giveaway (as well as someone seeing a fox about 1/4 mile away and me hearing one bark one night) and I have since been told that foxes often decapitate their victims very cleanly and don't carry them off to eat, as you might think.