Official line is he died of a myocardial infarction (heart attack).
I suspect he was assassinated. Conjecture. Power struggle. Flag at DPRK embassy in Beijing is not at half mast. No discussion of Kim Jong Un as successor. Fasten seatbelts. Most Presidents would cancel a Hawaiian vacation over something this serious. Going to monitor DPRK radio right now and hear there take. They took over two days to make it public. He will never see his 2012 when N Korea was to come of age.