Unless someone can tell me where he got this cyanide, then the investigation is not complete. There is a possibility that someone could have killed him using cyanide. If police can not answer the question where he got the cyanide, they are shortcutting, and have not solved this case. Its a controlled substance so there is a trail for trace elements of this gas. They can find out who bought it and where. There are professional killers who can reverse engineer crime scenes, and fool investigators. Perhaps not in this case, but can you answer the question , where did he get the cyanide? If not, then the guilty are walking away free.
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