Captive Student: A teacher, a student and secrets that became a cautionary tale

When Elizabeth Thomas entered public high school in Culleoka, Tennessee, she left one alleged nightmare for another.

Ultimately, she would be taken by a school teacher across state lines, setting off a nationwide manhunt.

A new episode of "20/20" looks at the story of Elizabeth Thomas which, miraculously, did not end in tragedy, with investigators laying out never-before-heard elements of the case.

Thomas, who had been home-schooled, claimed she had been physically abused as a child by her mother, Kimberly Thomas. She was ultimately removed from the family home and charged with child abuse and neglect. Kimberly Thomas denied those allegations but ultimately reached a plea bargain and the charges were expunged.

Elizabeth's father, Anthony Thomas, worked long hours in pest control to support the family and says he had no idea how bad things were at home.

"We had a lot of stuff going behind closed doors that shouldn't have [happened]," she told ABC News' Eva Pilgrim in a 2018 interview.

At her new high school, she met a health science teacher named Tad Cummins who initially acted as a mentor for her, before becoming her abuser.

"She was in his class," Elizabeth's father Anthony Thomas told ABC News in a 2017 interview. "And he began to help her make this transition from home-school to public school."

At a certain point, Elizabeth said she realized that their relationship was becoming inappropriate.

"He'd just be staring at me all during class," she told ABC News' Eva Pilgrim. "He was always eyeballing me, looking at me, sitting at our table."

"There was one time where he told me that I'd 'look nice naked,'" she said. "I realized, 'This is getting too far.'"

Out of school, they communicated via social media, with their messages becoming increasingly more explicit, she said.

Eventually, another high-school student witnessed Elizabeth and Cummins kissing and told the school authorities, who began to investigate.

Thomas and Cummins, who was married at the time, denied inappropriate contact but local authorities, who eventually got involved as well, were not convinced.

"I just got the impression from both of them that they were lying," said Marcus Albright, an investigator in the county's sheriff's department.

Cummins was suspended from the school for allegedly violating a no-contact order yet after five weeks of staying at home, in March 2017, he fled with Thomas.

"He always made the coffee, the night before we would go to bed," said his then-wife Jill Cummins. "He started telling me how to make the coffee and, and I was like, 'why are you telling me this?'"

He told his wife that he was leaving town for a job interview, Detective Albright said. Jill Cummins said that the next morning both his guns and $4,500 from their safety deposit box were missing.

Elizabeth also left her home early that morning with her bags packed, captured on the house's surveillance cameras. Her sister told ABC News that she told her "if I'm not back by 6, you need to come find me and call the cops."

It became clear to authorities that the then-50-year-old high school teacher had disappeared with his former student. At that point, both the FBI and Tennessee Bureau of Investigation got involved.

Authorities tracked their location to Alabama, then Oklahoma and eventually California.

Although they initially went to a commune in northern California, where authorities say they went by pseudonyms and pretended to be a married couple, they were eventually discovered two hours south in a town called Cecilville.

A local resident named Pete Cafferata had a suspicion that they might be the missing student and teacher who were by then making national news headlines.

"I had remembered this news story about a younger girl running off with an older man," he told ABC News. "I googled it, and it certainly looked like the guy."

Cafferata and another resident named Griffin Barry, who had let the couple stay in a cabin on a property where he worked, decided to call the police.

A SWAT team was sent in, and they arrested Tad Cummins. Elizabeth was reunited with her family after more than a month of being held captive.

Elizabeth said Cummins had used threats to get her to leave with him.

“He said if he couldn't have me, he'd kill himself,” she said. “Any time he threatened himself, he'd threaten my family.”

Thomas, who at one point had said to those close to her that she loved Cummins, slowly began to piece together the significance of the experience she had been through.

"I know he's a bad man," she told ABC News' Eva Pilgrim. "He only used me for one thing." She admitted to being manipulated and sexually abused.

During an interrogation with the FBI, Cummins finally admitted to sexual contact with Elizabeth.

He was charged with obstruction of justice and transporting a minor across state lines for the purposes of engaging in criminal sexual conduct, and sentenced to 20 years in prison.

Elizabeth told ABC News her dreams for the future are simple.

"To have a family and protect them," she said. "Not let them lead down the road like I was, and make them have a better life."

She is now married and a mother to a son.