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Dan Rivera -- the paranormal investigator on a creepy "Annabelle" doll tour who died completely out of the blue in the "haunted" Civil War battlefield town of Gettysburg -- had a history of heart issues ... and the coroner says his death was cardiac-related.

Rivera's autopsy results have been released by County of Adams Coroner Francis Dutrow , and we're told Dan died from natural causes. Getty

As we reported ... Dan was in Pennsylvania for his "Devils on the Run" tour -- hauling the infamous Annabelle doll across the country -- when he died July 13 in his hotel room.

The coroner tells us the Annabelle doll was NOT in the room when Dan died. Facebook / Comedian Matt Rife

We're told Dan had a known history of cardiac issues, which were consistent with the a

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