Mariska Hargitay reveals shocking family secret about her biological father

Mariska Hargitay has been keeping a family secret for more than 30 years.

At the Cannes Film Festival on Saturday, the "Law & Order: SVU" star revealed at the premiere of her documentary, "My Mom Jayne," that her biological father is former Las Vegas entertainer Nelson Sardelli. She was raised by late actor and bodybuilder Mickey Hargitay.

"I grew up where I was supposed to, and I do know that everyone made the best choice for me," she said. "I'm Mickey Hargitay's daughter. That is not a lie. This documentary is kind of a love letter to him, because there's no one that I was closer to on this planet."

Growing up, Hargitay always felt she was different from her siblings, the outlet shared. It wouldn't be until her 20s that she learned why.

When Hargitay was in her 20s, someone showed her a photo of the Italian entertainer. According to the outlet, Hargitay "immediately knew in her bones" that Sardelli was her biological father.

"It was like the floor fell out from under me," she said in "My Mom Jayne."

Mansfield died in 1967 from injuries she sustained in a car crash. She was 34. Hargitay was 3 years old and asleep in the back seat of the car when it rammed into a truck.

After suspecting that Sardelli was her biological father, she confronted Mickey. He denied it. According to the outlet, the encounter was "so shattering" that she never mentioned Sardelli to Mickey again.

Hargitay suddenly became fiercely protective of her father, the man who raised her and her siblings after her mother's death.

Hargitay eventually forged a bond with Sardelli, 90, and his daughters. But when it came time to tell her story, she said "many people" were hesitant about the film coming out. But in time, both families gave Hargitay their blessing. The documentary features interviews with Hargitay's siblings: Jayne Marie, Zoltan and Mickey Jr.

The Sardelli sisters "wept and wept" when they saw a private screening of the documentary in Las Vegas, the outlet revealed.

The film also helped Hargitay address the loss of her mother.

"I don't remember the accident," she said. "I don't even remember being told that my mom had died. I looked at photos, and I don't really remember anything until I was 5."

WATCH: QUEEN ELIZABETH 'FELT SORRY' FOR MARILYN MONROE: AUTHOR

"When I would hear that fake voice, it used to just flip me out," Hargitay recalled. "'Why is she talking like that? That's not real.' [But] my dad would always say, 'She wasn't like that at all. She was like you. She was funny and irreverent and fearless and real.'"