The Medical Examiner is ruling Leandro's death an accidental overdose.
As we reported ... Robert's 19-year-old grandson was found dead of a suspected overdose last month ... with cops saying investigators found one bag of cocaine, one straw with residue, two blue pills and seven rectangular tablets believed to be Xanax.
Police said Leandro bought counterfeit oxycodone and Xanax just days before his death ... and the fake oxycodone pills were laced with fentanyl.
The official cause of death also lines up with what Leandro's mother, Drena De Niro, has said on social media, "Someone sold him fentanyl-laced pills that they knew were laced yet still sold them to him."
Meanwhile, the woman who allegedly sold Leandro the drugs has been arrested and charged ... and she's facing up to 20 years behind bars.