So sorry for your loss. My suggestion would be to put up a camera and see what's been lurking and that is probably the killer and it will be back and then you can form a plan on how to deal with it. Good luck...
http://www.poultrydvm.com/featured-infographic/tips-for-protecting-poultry-from-predators
http://www.poultrydvm.com/featured-infographic/tips-for-protecting-poultry-from-predators