I really pray they will do a thorough autopsy. Cyanide is fairly easy to detect as there is a distinctive smell associated with it. Ricin is much harder as it only takes a tiny amount to kill. Potassium chloride can be injected and will cause the heart to stop, particularly in someone who already has heart issues. It can be detected if they're looking for it, but if they assume "natural causes" and don't do a thorough autopsy, it is easy to miss potassium chloride and many other chemical means of killing.

How thorough the autopsy is may depend on his family. If they push against it, the medical examiner may just do a cursory exam, though I think they'll have to do one because of the circumstances and Breitbart's age. However, if Breitbart was having serious enough issues that his life was in danger and his family knew it, they may autopsy just enough to "verify" what they believe is the cause and then let it go. That may depend also on how stretched the ME's office is.