Oh no...We've had that happen too. It's horrible and the guilt really can get to you. Just remember, EVERYTHING eats chicken. We have the Fort Knox of chicken runs and we still end up with a few who were picked off each year. It's really frustrating when the predators just kill them and leave them, I mean, you gotta eat but can you just take one eat it and leave the leave the rest?...geeze, inconsiderate little buggers.
I would guess a fox or coyote. They tend to kill all they can and carry off whatever they can hold. We've had up to 20 taken out just like this. It's kinda heartbreaking but it definitely happens.
I'd be happy to share any tips on securing your coop if you'd like. I think our chickens are a little cocky because they know nothing can get to them. To be fair...we haven't had a bear try his luck with it but our dogs chase off most everything that doesn't already live here.