This is just what I heard (Recently moved to Saint Joseph): The couple was found in their garage/car and died for asphyxiation. The rumor around here is that their son was suspected of having had something to so with their death some how but it was never pursued by investigators. Aside from that, Dr. Prokop was (and is) known to be the best pediatrician around and was loved by the familes he served. Rumor has it, the son cashed in and moved out of town.
Not sure if that's helpful, but I came across your post the same day I heard about their deaths. People still talk about the beloved Dr. Prokop and the mysterious way he and his wife died.