If they all died with no foul play, it could be carbon monoxide from a faulty heating unit.
Definitely a possibility. Weird Al Yankovic lost both of his elderly parents to carbon monoxide poisoning.
That was my first thought. A clogged flue for the fireplace or woodstove, or faulty heating duct. I also thought about food poisoning, but they would have to feed the dog the same food, which a lot of people do. The “No foul play” line is is premature from my perspective, if they don’t know the cause of death, how can they rule out foul play?
My thoughts exactly. Probably Carbon monoxide poising. Dad had some friends killed by it in Farley NM way back in the 1960s. That is why I sleep with a window slightly opened even in the coldest nights and will NEVER use a kerosene heater in my house.