Leandro De Niro-Rodriguez's official cause of death has been ruled as an accidental drug overdose.
The New York City chief medical examiner's office confirmed to People the 19-year-old's July 2 death was from the "toxic effects of fentanyl, bromazolam, alprazolam, 7-aminoclonazepam, ketamine, and cocaine."
Taking to Instagram days after his death, Leandro De Niro-Rodriguez's mother, Drena De Niro, wrote, "Someone sold him fentanyl laced pills that they knew were laced yet still sold them to him."
"So for all these people still f---ing around selling and buying this s---, my son is gone forever," she added.
Sofia H. Marks, 20, has since been arrested in connection to Leandro De Niro-Rodriguez's death and stands accused of selling the drugs to him.
Shortly after his grandson's death, Robert De Niro, 79, released a statement saying he was "deeply distressed."
"We're greatly appreciative of the condolences from everyone. We ask that we please be given privacy to grieve our loss of Leo," he added.
Drena De Niro, meanwhile, wrote a tribute to her son upon announcing his death on Instagram.
"My beautiful sweet angel. I have loved you beyond words or description from the moment I felt you in my belly," she wrote.
"You have been my joy my heart and all that was ever pure and real in my life," the actor, 55, continued. "I wish I was with you right now. I wish I was with you. I don't know how to live without you but I'll try to go on and spread the love and light that you so made me feel in getting to be your mama."
She also marked one month since his death with a heartfelt post on Instagram, writing: "I think of you and long for you, every minute of every day. Never forget how happy you made so many people just by being here and being beautiful kindhearted you. Your legacy is Love."
If you or someone you know is struggling with substance abuse, please contact the SAMHSA helpline at 1-800-662-HELP.