What's killing my chickens

So, three times in the last three weeks something has gotten into my chicken coop and killed a chicken. A chicken, just one each time. Dead in the coop, head missing and ass end eaten out. Each time, same way. I've gone over the coop to try and make sure the loose spots were tacked down, seams closed, etc. The first time I found a hole under the backside, dug under the buried chicken wire. Plugged that up with rock and pallets good for more than a week. Then the other day, another one killed, same way. That's when I found a couple of loose seams and one top edge kinda open. stapled and nailed and tacked down everything I could, and today another dead one. They have been fine in this same coop for over a year. How and why now, and what kind of varmint kills just one at a time and takes off the head and only eats from the backside?