Sorry to hear that! It might be that he ate some poison or toxic substance that caused massive bleeding to his intestines. If you have other dogs or small children, it is probably a good idea to take his body to a vet. A vet can let you know if there is anything in your environment to worry about.
It would be very difficult to say .. But, it could be something toxic that he had gotten into, that caused internal bleeding. Or he had been shot, or severely wounded somehow .. There are just too many things it 'could be'. I would be very curious to know what caused this, only so I could prevent it from ever happening to any other pet.
I am so sorry for you loss...
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