I am so very sorry for your loss, i know how hard it can be. If you find anything out from an autopsy please get back to us.
And i am so relieved to hear that your other dog does not have distemper,
As a word of advice, Here's a list of food items that can harm and possibly kill your dogs in the future:
since it's always good to know.
Avocado, Alcohol, Onions, Garlic, any caffeine product, grapes, raisins, gum, dairy products, macadamia nuts, candy, chocolate, fat trimmings, peaches, plums, raw eggs, raw meat, fish, salty foods(i.e chips, pretzels), sugary foods and drinks, yeast dough, and bones. Bone's can easily be choked on, and can shred in their stomach.
Again, i am so very sorry for your loss, and my heart goes out to you and your family.