Robert De Niro will forever mourn the death of his late grandson Leandro De Niro Rodriguez who died of an overdose in July. He was 19.

"It’s just a shock," he told People magazine. "[I] never thought it would happen."

One month after Leandro died, New York's Office of the Chief Medical Examiner confirmed to Fox News Digital that his cause of death was due to the "toxic effects of fentanyl, bromazolam, alprazolam, 7-aminoclonazepam, ketamine, and cocaine." 

ROBERT DE NIRO'S GRANDSON'S CAUSE OF DEATH REVEALED

Robert De Niro wears brown jacket for portrait snap

Robert De Niro was in "disbelief" upon hearing the news his grandson Leandro died in July due to a fentanyl overdose. (Gareth Cattermole)

De Niro remembered feeling "disbelief" upon hearing about Leandro's death. Leandro is the son of De Niro's oldest daughter, Drena, with ex Diahnne Abbott. 

The "Killers of the Flower Moon" actor adopted Drena when he and Diahnne married in 1976. They divorced 12 years later in 1988. 

ROBERT DE NIRO'S DAUGHTER POSTS TRAGIC TRIBUTE TO HER 19-YEAR-OLD SON: ‘YOU DIDN’T DESERVE TO DIE LIKE THIS'

"My grandson, on July 2nd of last year, he was … he overdosed with fentanyl," De Niro said. "He was just about … going to be 20. It's awful."

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"And I just then started thinking about all the things I could have, should have done maybe with him. I don't know if that would've made a difference. And so that's always playing through my mind."

Rodriguez's manner of death was ruled an accident, according to the medical examiner.

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De Niro added, "It's not a good thing for anybody. It shouldn't have happened."

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Drena, an actress and filmmaker, worked alongside her son in multiple projects, including "A Star is Born."

Sofia Haley Marks, 20, was arrested in connection with Rodriguez's death in July. Marks allegedly sold Rodriguez drugs before he was found dead July 2 in his Manhattan apartment. 

She appeared in a Manhattan federal court two weeks later on three federal narcotics charges and agreed to remain behind bars until she requests bail at a later date. Prosecutors said at the time that they would oppose a request for her release.

Robert De Niro in a black shirt sits next to his daughter Drena De Niro

Robert De Niro adopted daughter Drena from his relationship with Diahnne Abbott. (Dimitrios Kambouris/Getty Images)

"I’m deeply distressed by the passing of my beloved grandson Leo. We’re greatly appreciative of the condolences from everyone. We ask that we please be given privacy to grieve our loss of Leo," the Academy award-winning actor said in a statement shared with Fox News Digital.

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