I'm not sure what the predator was. The coop is in a wooded area so most of the time the chickens were free ranging well under cover. The first two died deep enough under some brush that I am pretty sure it wasn't a hawk or other sort of flying predator. Also, for these two chickens I only saw feathers; no body. So whatever it was that ate these chickens either carried it a ways off or ate the whole thing.
The most recent bird was decapitated right next to our road. This was unlike the other two because the rest of the body was there. It was just missing a head (the body was a little mutilated, but still there). I'm not sure what killed this last chicken, but I am thinking it was something different than the animal that killed the first two.
At this point I've decided to keep all the birds in their cage for a while. Before this little killing spree I had let them free range in the day and locked them up at night. If I can't find the predator after a few weeks, I may start letting them try to head back out. Especially now that I have a rooster that may help protect them and alert me.