That's horrible. I'm so sorry for your loss. It sure sounds like a raccoon to me. As stated by @Cryss, they will reach in, pull the bird's head or wings through the wire, bite them off and either eat the head or leave them.
An Amish neighbor had it happen with pullets that he had in a grow out pen in his yard. It took him 3 or 4 months to get the culprit and it was a big raccoon. He introduced me to their habit. When a chicken leans against the fencing and falls asleep, the raccoon will reach in, snare the neck and pull it through. The bird doesn't know what hit them.
My advice is to get a Hav-a-heart trap and set it. The varmint knows where it can get a free chicken dinner and will be back for seconds.
Take care. You have a lot of work ahead for you fortifying your coop and run. But it will be worth it.