It sounds like a raccoon to me. Raccoons bite the head off to debrain the bird. It causes the feathers to come out quite easily. They usually then go straight to the middle of the back and bite a deep hole. They are more than able to carry one off too. They quite often travel in pairs or as a family.
We've had two chickens (one hen and one BIG rooster) survive raccoon attacks. We actually heard the commotion in our hen house about 1:00 in the morning one morning. We FLEW out there and caught the scoundrel. He had bitten our hen's head, but did not bite it off. Her head swelled huge. She was black and blue as if she had been beaten. She ended up losing one eye. We called her Miracle after that.
He had already killed two others of our hens. He had tried to stuff one under the fence to his waiting mate.
Raccoons can be tough predators. We now have a critter-friendly trap that we bait with canned cat food. When we catch one (and we catch them often), we haul them far, far away to a State Park area where there is fresh water, heavily wooded areas, and NO livestock close by. There are those who believe this isn't right, because you are taking them out of their territory and putting them in other coons territories. But I cannot kill something that is just busy being what God created it to be.
By the way, I've heard it said that you have to take them at least 10 miles or more away or they will come right back (or die trying).
Hope I didn't type too much. Good luck with your remaining bird.
PS Raccoons can scale ANY kind of wall, I think. I've seen them go straight up the side of our hen house. If there is a hole in a fence or around your coop ANYWHERE, they WILL find it sooner or later. I think they must have a keen sense of smell. Anyway, let us know if you find out for sure what it was.