Cops Say Robert De Niro's Grandson Took Fake Oxycodone Pills Laced with Fentanyl

Cops say Robert De Niro's grandson ingested fake oxycodone pills laced with fentanyl, and even though his alleged dealer knew the pills could kill ... she sold them anyway.

According to legal docs, obtained by TMZ, investigators found several pills at the death scene of 19-year-old Leandro De Niro. They say they went through the teen's phone, and discovered he had exchanged texts with a 20-year-old woman named Sophia Marks about purchasing counterfeit oxycodone and Xanax just days before his death.

Most disturbing, cops say during one exchange Marks asked, "do you really need them? I don't wanna kill you." She continued, "I just don't like serving u them cuz they not script."

Eventually, the two struck a deal for 3 of the fake oxycodone pills and two tablets of Xanax for $105 ... and Marks allegedly texted (about the Oxy), "don't go overboard with these."

The drugs were delivered by car to De Niro around 9 PM July 1, and at 1:50 AM July 2 cops say Marks texted the teen, "u good?" ... De Niro never responded.

De Niro was found dead of a suspected overdose on July 2 ... investigators found one bag of cocaine, one straw with residue, two blue pills with "M" and "30" (the oxycodone) and seven rectangular tablets -- believed to be Xanax.

Shockingly enough, cops say Marks continued to sell after De Niro's death -- selling an undercover agent 25 pills for $500 on July 9 ... allegedly telling the undercover, "please be careful with these. don't do more than one at a time. my friend just died."

Investigators say further analysis revealed both the pills found at the De Niro scene as well as those sold to the undercover all contained traces of fentanyl.