I'm assuming no tracks were found. If so, that would help. I have heard of chickens dying of frieght, too. That would open the door to other culprits other than dogs, such as a coon, fox, weasel etc. It's more believable that these wild carnivores would only go after one, anyway. I can't imagine a tiny weasel, or a racoon running down that many chickens. That would expend too much energy.