He could have gotten maced or pepper sprayed. That can shut down the lungs in an allergic reaction. Or he might have gotten into a toxic chemical somewhere causing his system to shut down.
The only way to know for sure what happened to him would be to have the vet do a necropsy (like a human autopsy) to find out cause of death.
Be very careful handling his body if you suspect toxic chemicals might be the problem.
Its very hard losing a healthy dog so suddenly. If you have not seen www.petloss.com and the rainbow bridge story that might bring you some comfort.
Hope this helps you!
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