I am very sorry that this happened to you. I have experience withis this type of thing and have lost chickens to dogs, foxes, ringtail cats, and badgers.
From the look of it, I would say a dog. A few years back I gave my neighbor 13 six month old chickens to get him started. Well, that evening his dog tore into his wire enclosure and killed every one of them. Did not eat them just savged them and left. He then came up to my place over a mile away and broke into the chicken yard and killed five more. Some dogs just go into a frenzy and kill this way. As they say out here, "once they get the taste for it"
If the carcasses were more or less intact, they still had their heads, then you can rule out ringtail who kill by biting off the heads.
You could have foxes out there but they do not usually kill more than one or two and are likely to drag them away before eating them.
Other predators will usually consume almost the whole bird.
This does sound like a dog, and it might be hanging around to look for more, but if you get set up again, it might return for act two.
The best way to be safe and have peace of mind is to put them up at night and close them in their house. Early morning is prime time for attacks, usually just before sunup.
So sorry, I know how distressing this is even to lose one this way, but the whole flock. Well Matt, it is life and death in God's wonderful creation.
God Bless you and the kids. (Wife too :farmer:)