If the bites didn't come from the mastiffs, then where did they come from? A strange dog that just happened to be in the house? That doesn't make sense. The mastiffs would have taken what ever attacked the man and killed it.
I am sure one of the mastiffs are to blame for the bites. Could have been one of the dogs, two, or from all of them. If it was originally thought that he died from the attack, then I am sure the bites were pretty bad. Not just a scratch here, a little puncture over there, and a couple small lacerations over there. I am not saying that the dog or dogs were trying to kill the guy, but obviously they were deliberately inflicting bodily harm even though the heart attack killed him.
Just my opinion.