I'm glad you guys solved the mystery and hopefully it won't happen again. Sorry you had to go through that it sounds sad. We almost had a similar issue on our hands when a dog showed up on our property January 1st. He was looking at the ducks like they were a mighty fine snack!
Side note: I had what I think is an awesome idea a few weeks ago. What if you took one of those metal dog crates (used one preferably that someone is selling for cheap) You can attach some hardware cloth or chicken wire around the bottom, potentially add a covered part and it's an animal tractor!