Sierra Barter has wondered if her step-grandfather could have been a serial killer.

“I remember my mom mentioned she Googled the Santa Rosa Hitchhiker Murders,” the 32-year-old told Fox News Digital. “She was freaked out about it. … And I was just like, ‘This is too much. I can’t think about this right now.’

“But then, maybe a couple of days later, I went, and I looked at it – really looked at it,” she shared. “It shook me. It is not a fun thing to imagine that somebody that you spent time with or that you have photos of yourself with … could have committed these kinds of crimes. … It’s an unsettling feeling.”

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The California woman’s quest to uncover the truth is being explored in a new four-part documentary on Max, “The Truth About Jim.” It details how Jim Mordecai, a respected high school teacher and star football player, had a history of alleged physical violence and sexual assault against women and girls. 

The docuseries features interviews with Mordecai’s daughters, extended family members, a private investigator and law enforcement.

Mordecai died in 2008 at age 67. Barter was 15 at the time. But there have always been rumblings about who Mordecai really was. And when Barter finally began asking questions, she wasn’t prepared for the answers.

“After he died, that’s when I learned the truth about what caused my mother to have a separation from my grandmother,” she said. “Then it started to make sense about why there was so much division in the family.”

Growing up, Barter always knew that her mother, Shannon, detested Mordecai. She even went a full decade without speaking to her mother Judy, Mordecai’s third wife. 

“It really started with Sierra talking to her family about Jim,” director Skye Borgman told Fox News Digital. “That’s when they opened up to her about how evil he was to so many women. … Jim’s family was traumatized. … And it was hard to hear stuff like that. But it was also very humanizing and very brave.”

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The documentary alleged Mordecai repeatedly raped one of his stepdaughters from the time she was 13 years old. Other family members and alleged victims claimed the patriarch terrorized them with threats and violence. Some women in the documentary alleged Mordecai viciously bullied them and that he preyed on teen girls.

“Sitting down with my grandma was hard,” Barter admitted. “I’m very, very close to her. She was always milk and cookies grandma … but it was difficult to hear these things about someone I love so much. … But it was also such an honor to get to know her as a woman and understand how strong she was and what she went through.

“Hearing my aunt’s stories had a huge impact on me,” Barter added. “We were thankful we were free of Jim at that moment. … It was hard to hear how these women, women I shared giggles with, who dressed me up like a doll and all of that, were enduring such suffering behind closed doors. It was heartbreaking. … It was devastating to hear what was taken from them at the hands of their father.”

Through conversations with loved ones, Barter made a disturbing connection between Mordecai and the Santa Rosa Hitchhiker Murders. The series of at least seven unsolved homicides involving young women took place in the North Bay Area between 1972 and 1973. Two other suspects include serial killers Ted Bundy and the Zodiac Killer. Three of the victims had been bound and two were hogtied.

The documentary alleged Mordecai frequently threatened to hogtie women and “throw them into a ditch.” He also allegedly threatened to kill one of his ex-wives when she asked for a divorce and said, “If you try to get the girls, I will slit their throats,” referring to their children. 

Mordecai also kept a collection of cheap, random women’s jewelry.

The documentary revealed Barter handed over some of Mordecai’s personal belongings containing possible DNA to the Sonoma County Sheriff’s Office. The investigation is still ongoing, it noted.

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“These cases are very old,” Barter admitted. “There’s a lot of evidence that isn’t there anymore. There’s a lot of follow-up that needs to happen. A lot of people have passed away. But I’m still hopeful. Maybe there’s a chance we can finally get some answers for these families.”

At one point in the documentary, Barter wondered if there was a possibility Mordecai could have been the Zodiac Killer. The identity of the infamous murderer, who killed at least five people during the ‘60s, is still unknown today. However, the documentary showed that there wasn’t enough compelling evidence to link Mordecai to the crimes. Her theory ultimately unravels.

In 2021, a team of specialists who investigate cold cases, known as The Case Breakers, claimed the Zodiac Killer was Gary Francis Poste, who died in 2018. However, no one has ever been charged or identified in the murders.

At the time, Fox News Digital reached out to the Vallejo Police Department and the FBI and unsuccessfully tried reaching out to relatives of Poste. The San Francisco Police Department said it was unable to speak about potential suspects in the Zodiac case because it remains an ongoing investigation.

Arthur Leigh Allen, who has been frequently listed as a suspect, died in 1992.

Barter is still waiting for answers. But she feels free of Mordecai and his shadow, she said.

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“I’m just really happy I gained my family back,” Barter said. “It’s not fun to have a fractured family. … But I’m grateful that I don’t have to live with Jim lurking in the sphere anymore. … And I’m so grateful to be surrounded by a group of women reminding me to speak up.

“There’s not a lot of Jim’s childhood that survived,” Barter reflected. “I can only assume that people who are hurt tend to hurt other people. But I don’t want to make excuses for him. There is always an opportunity to get help, which he did not choose to do. I’m not sure why he did the things that he did. I think he was feeling powerless, and that’s why he took his anger out on people and tried to take their power. 

“But, frankly, I’m not so sure if I want to know why. I’m just thankful that these women are now speaking up against him. He’s not able to rest with the idea that he was this great man because we all know that he was not.”