Cops have a woman in custody who they believe is directly linked to the overdose death of robert denirothe grandson of, Leandro De Niro Rodriguez …and TMZ has learned that authorities are also on the case.

Law enforcement sources tell us the 20-year-old was arrested in New York on Thursday by a joint NYPD and federal government task force – and they believe she sold drugs to Leandro before he died … which is what her mother said was due to a fentanyl DO.

We’re told the suspect is being treated at the moment, and we don’t yet know what charges she will face.

As we reported, the 19-year-old was found dead in an apartment on July 2. He appeared in the 2018 film, “A Star is Born”… playing the son of bradley cooper‘George “Noodles” Stone’s best friend, played by Dave Chappelle. Leandro also appeared in “Cabaret Maxime” in 2018.

Law enforcement sources told TMZ that Leandro’s death was being investigated for an overdose. We were told the teenager was guarding his home and drugs and drug paraphernalia were found near his body.

While authorities have yet to release the official cause of death, his mother, Drena De Niro, said last week, “Someone sold him some fentanyl pills that they knew were mixed up, but sold them to him anyway.”