I would say raccoon since it seems like the predator grabbed from the neck and maybe ran off once the bird died. It could also be a weasel since they like to grab the jugular and pull them out.
I'm really sorry about this and hope that the rest of your ducks are safe. If you want to keep them safe from other attacks, build something around their home where the raccoons or weasels can't reach. I suggest using chicken wire around their home and then surrounding it with some barbed wire so that the ducks won't get hurt, but the predator will. If this doesn't work, set up some traps. I don't like harming other animals but sometimes you gotta do what you gotta do in life.
Hope this helped!