Apparently they're investigating whether or not he had good or "bad" heroin. It seems that a lot of people have OD'd up there shooting a mixture of heroin and fentanyl. For some reason, assholes are selling the mixture without telling their customers what it is, and people OD due to the considerably higher potency. Honestly, killing your customers is a remarkably shortsighted business strategy.
A neighbor also reported seeing him yesterday looking ashen and generally terrible. I suppose it's also possible there was an underlying medical condition. I'd certainly bet on the overdose, but it just seems odd to me that somebody with 20+ years of heroin experience would OD with a still full needle in their arm.