Thanks everyone, and due to everyone suspecting he was poisoned, we're sending his body to be examined. He was a very loved dog, everyone at the barn loved him. Everyone wants to know what really happened to him, so soon we should know.
I'm very sorry for your loss. I agree with coffeeaddict in that I would keep my other dog inside. My little sister lost 2 labs last year. One from poison, the other went blind and died after he had been maced. It was her husband's grandparents who did it. They admitted it her MIL. She said that there was nothing she could do. Said she couldn't take them to court because her hubby didn't want to do that to his grandparents.