My sympathies to the family and friends of the deceased and stricken.
But heart attack can be a symptom of heat stroke. And with the temperatures we had on that day coupled with the humidity it's very possible it was still heat related. And one does not have to be dehydrated to get heat illness.
We saw it with the couple who died in Joshua Tree last year who were only able to walk a few miles in temps just over 100º and we saw it recently with the ultra running who was obviously quite fit dying in Death Valley.
Hiking in the heat is very dangerous and should be avoided whenever possible. And if you're going to go out there, be very aware that maintaining core body temperature is as important as staying hydrated. Keeping the direct sun and its radiant heat off of your body is an important way to do this.
