As Law & Order: SVU’s Olivia Benson, Mariska Hargitay is a pro at solving complex cases. Now she’s sharing a long-running mystery in her own family for the first time. In My Mom Jayne — her new documentary about her late mother, the actress Jayne Mansfield — the 61-year-old revealed a secret she’d kept for decades: At 25 years old, she learned that the man she thought of as her dad, Hungarian bodybuilder Mickey Hargitay, was not her biological father — but rather a former Las Vegas entertainer named Nelson Sardelli.

The film, Mariska’s directorial debut, is her attempt to discover the truth about Jayne, a beloved Hollywood bombshell killed in a tragic car accident when Mariska — who survived the crash — was just three years old. “I don’t remember the accident. I don’t even remember being told that my mother had died,” Mariska told Vanity Fair. “I look at photos, and I don’t really remember anything until I was five.”

After she confronted Mickey about her parentage, he insisted that he was her real father — and the two never spoke of it again. He died in 2006. Undeterred, the actress went to see Nelson perform in Atlantic City, NJ. “I’ve been waiting 30 years for this moment,” she recalled him saying when she introduced herself.

The confirmation didn’t bring relief. “I was like, ‘I don’t want anything. I don’t need anything from you… I have a dad,’” she told the magazine, adding, “I wanted to be loyal to Mickey.” She struggled, Mariska confessed, especially after learning that Nelson had two other daughters. Still, she continued to keep quiet. It feels cathartic to get it out, she shared, after “knowing I’m living a lie my entire life.”