I remember what my grandfather did after my dad's black lab puppy killed a chicken. he tied the dead chicken to the back of the neck and head of the dog for a couple of days in such a way the dog could not get it off. Ater that the dog would not go near a chicken. I am sorry to say but chickens are a little low on the food chain. I have lost chickens to pigs, cats (when they were small) horses, especially welsh ponies, rats, racoons, possums hawks etc.
I am sorry to hear that you lost the entire flock, but I would expect that they did not eat all of them. Chickens will travel a long way. We have wild bantaams that I see that I see from time to time that cover a 4 to 6 square mile area around us.