What's killing my chickens?

We came home last night to find four of our chickens dead and the rest were terrified huddled together on perch. Our chickens are six to eight weeks old Two of the dead did not have any visible injuries except their necks looked broken the white leg horn was in a box the other one a buff orpington was next to the other two red rhode islands in a corner. The two rhode islands were pretty mangled one was almost completely eaten with only the head and one wing left the other rhode island's guts were hanging out of stomach. There was no blood anywhere except for the second rhode islands head. We cannot find an exact entry or exit point and our coop is completely covered top to bottom. We have dogwire six feet high with chicken wire two feet high around bottom and nets over the top and two feet down the sides they have part of our building for a shelter area it's separated from the rest of building by wood, roosting boxes, fence and net. We have no idea what animal did this or how they got in we do believe it happened around dusk please if you have any knowledge on this or suggestions we really need them we want to protect our other chickens but do not know where to start.

I see that you made no mention of using hardware cloth as part of your secure fencing. The things you did mention, dog wire and chicken wire aren't going to stop a lot of chicken predators, especially when they are so small and young. Rats, mink, weasels, and raccoons are just a few that could easily penetrate your defenses. If you can get a few fingers in a space, a raccoon can get it's whole arm in. If a Raccoon can reach a chicken through the fence, it's as good as dead. They can also shred chicken wire. It's not a good choice.

Thank you we will check on the hardware cloth.