It’s natural for an adopted person to be curious about their birth parents — but for Gary Stewart, his search might have led him to one of the country’s most infamous serial killers.

FX’s four-part docuseries “The Most Dangerous Animal of All,” (March 6 at 8 p.m.), based on Stewart’s 2014 book of the same name, follows him as he searches for his birth parents, uncovering information that leads him to believe his father was the notorious “Zodiac Killer” — the still-unknown murderer who operated in Northern California in the 1960s and ’70s, taunting the Bay Area press with letters of his deeds.

“The wanted sketch [of the Zodiac Killer] just made my heart stop for an instant, because I had seen that face before and it looked just like my father,” says Stewart, 57, who grew up with his adopted parents in Baton Rogue, La., where he still lives.

Stewart says that, although he wanted to meet his birth parents for his entire life, he didn’t meet his mother, Judy, until 2002. “I thought maybe me searching would hurt somebody’s feelings,” he tells The Post. “Fortunately, I didn’t have to. My biological mother came looking for me and we connected for the first time in 2002.”

Coincidentally, Judy’s second husband was a homicide inspector in the San Francisco police department and had worked the Zodiac case.

“He was deceased when my mother found me,” says Stewart. “But when I told her I wanted to find my father, she went to her husband’s old work colleagues and her friends in the San Francisco police department and asked them. It took several months to find my father’s files. But the information they presented to me was that there was information in that file that they were not willing to share with me — and that was pretty bad stuff.”

“Bad stuff” doesn’t always mean murder, but as the premiere episode of “The Most Dangerous Animal of All” reveals, Stewart’s birth father, Earl Van Best Jr., was a troubled 27-year-old who began a relationship with Judy in 1961 when she was just 13, for which he was later incarcerated.

“For the longest time my motivation was because I wanted to meet him, and hear his side of the story,” says Stewart. “And I wanted him to see me. I wanted him to know that I was a successful businessman and a graduate of Louisiana State university, an electrical engineer, and that he had a grandson. And if he had his side of the story, I wanted to extend forgiveness to him for what he had done to me.

“I wanted to tell him, ‘Look, it’s okay, maybe the greatest thing you did for me was to give me away.’”

Then, in 2004, Stewart was watching a cold case show about the Zodiac Killer hosted by Bill Kurtis. The wanted sketch of the murderer made him think he recognized his father. That sketch, combined with his father’s crimes, timeline and location all lined up with the Zodiac case.

“Do we really know the genetic effect of being related to a serial killer? How does that play out in a few generations?,” Stewart says. “[I wanted to take] that dysfunctional cycle of my father’s life, and use the new beginnings from the Stewart household to give my son and grandchildren an opportunity to walk away from this and never have to be concerned with what kind of blood they have.”

Stewart says making the docuseries helped him find some measure of closure.

“Every time I’ve reached a milestone in my search for my father, even now knowing he’s deceased, I always try to tell other people in hopes of convincing myself that I’ve got closure,” he says. “I think my book ended [by saying] ‘My father abandoned me so many years ago, maybe now I can abandon him and walk away.’ And that was six years ago. And I have not abandoned him. We spent the last year and a half filming this documentary…but this time, I’m over it.

“I’m done with it.”