So many people who lose chickens to predators are naturally interested in what possibly killed them. Normally, unless there is left behind some obvious evidence, it is really just guess work and the real problem is that once a predator like a fox, coyote, raccoon, bobcat, etc. find your chickens they will keep coming back until there are no more chickens or they are otherwise detered. Your only hope at this stage is to build a predator proof run and do not let them free range unless you are there to watch over them. You can set up traps and see what you get, but even then it is not a sure thing.