I suspect a dog - especially given that you know them to be around. I second the electric fence if you are free ranging them. Otherwise, you'll want to keep them closed in a secure run. (Even the electric fence does little for hawks and the like. But there are some strategies that can help if you go that route. At least with hawks they usually only take one at a time.)An electric fence would be your best bet. Meanwhile, whatever killed them will continue lurking, and a camera may help you identify the killer. I'm sorry for your losses.