Cops have a woman in custody who they believe is directly tied to the overdose death of Robert De Niro's grandson, Leandro De Niro Rodriguez ... and TMZ has learned the feds are now on the case too.
Law enforcement sources tell us the 20-year-old woman was busted Thursday in NYC by a joint NYPD and federal task force -- and they believe she sold drugs to Leandro before his death ... which his mother has said was due to a fentanyl OD.
We're told the suspect is being processed at this moment, and we don't know yet what charges she will be facing.
As we reported, the 19-year-old was found dead inside an apartment on July 2. He had appeared in the 2018 film, "A Star is Born" ... playing the son of Bradley Cooper's best friend, George "Noodles" Stone, played by Dave Chappelle. Leandro also appeared in "Cabaret Maxime" in 2018.
Law enforcement sources told TMZ Leandro's death is being investigated as an overdose. We're told the teen was house-sitting, and both drugs and drug paraphernalia were found near his body.
While authorities have yet to release an official cause of death, his mother, Drena De Niro, said last week, "Someone sold him fentanyl laced pills that they knew were laced yet still sold them to him."