Woke up to my entire flock dead this morning

All my chickens were killed last night. Of my six chickens, one was partially eaten, and the rest were bitten on their neck/shoulder and otherwise intact. What could have killed them? My fence wasn't damaged, but the little door to the run was open (that's on me, I know). We didn't seem to have any predators around so I got a little lazy on that one. Our neighbor has a cat that hangs around, but we haven't had problems before.