I concur with all other posters. It was likely a raccoon. We caught a massive boar coon at Halloween this year after he killed a half a dozen of my best chickens. All of the damage was the same- necks stripped of feathers or gnawed through and the main body cavity eaten.
I am a newbie and want to prevent this. Were your chickens out of the coop? Was the coop opened by the coon, or a hole dug? Did this happen in the day or night? This info will help me building my coop. I want an open bottom run but if the coons dug under I will take my dad's advice and make it a screened bottom. I am so sorry this happened to you. I have a two year old and four year old and while they need to learn about these kinds of things I'd like to prevent this from happening if I can.