I feel for your loss as we just had the same problem.
I found 15 of my flock of 25 dead yesterday afternoon. All of them were scattered all around, some inside the enclosed coop/run and others outside the coop/run in an enclosed yard area. Some mutilated having their breast and insides eaten, others just a head missing, and in between. We had no idea what could have done this. This morning as I before going to work, I went out to check on the remaining 10 that I had locked up in the coop/run. There I found the family of 6 young raccoons finishing off the 10 remaining birds. I found where they entered as 1 of them made his/her escape. They tunneled under the door. This was the ONLY place I did not protect my coop/run by putting hardware cloth/wire on the ground outside the door as I did all the way around the coop/run. Only myself to blame for this costly oversight of mine. When I built my coop/run I knew we have all types of predators (foxes, owls, hawks, eagles, raccoons, opossums, skunks, coyotes, bobcats, many species of snakes, and who knows what else) so I built it with the materials and a design that we would not have to worry about predators. We are going to start back over but not before I make the coop/run totally predator proof. We have gone for almost 3 years without any problems until now. We live in a rural, country, setting in S.W. GA and having lived here at this location for 15 years we have seen and fed all the predators mentioned (except for snakes) along with 6 cats we have as pets. To note our cats never bothered our chickens, in fact 2 our our youngest cats, I actually found trying to roost with the chickens and the chickens fussed until I got the out of the coop, and I found the cats eating out of the feeders and the chickens eating the cat food in the garage. I hope you find out how they gained access and correct the weak spot and rebuild your flock as we are going to do.
