Zenya Hernandez didn't think it was strange at first when her 14-year-old daughter Abigail Hernandez didn't come home from school on Oct. 9, 2013.
"Well, right away, I was like, 'She should be somewhere,'" Zenya Hernandez told ABC News’ “20/20.” “'She is probably at school with her friends.’”
Zenya Hernandez, a nurse, enjoyed a happy life in North Conway, New Hampshire, raising her two daughters: Sarah and Abigail, known as Abby.
“Abby always has been very kind, loved animals. (She was a) very cheerful, happy child," Zenya Hernandez said.
As a new high school freshman, Abby Hernandez studied German and robotics and loved classic rock. Zenya Hernandez said that her daughter was very athletic and that they hiked a lot together.
“It was a good life,” Zenya Hernandez said.
That’s why when Abby Hernandez didn’t answer her mother’s text messages, Zenya Hernandez started to worry.
“I was like, ‘OK, I’ll go to school.’ So I get to school, and then the librarian says, ‘I saw her leave.’ That’s when I got very worried,” Zenya Hernandez said.
Abby Hernandez had never not shown up at home before, her mother said.
“My thinking was, ‘What if she had an injury? Broke her leg. Appendicitis. Who knows? Got hit by a car,’” Zenya Hernandez said. “I called the hospital at that time. They have not seen her. At that point, (I'm) kind of getting frantic actually.”
But Zenya Hernandez didn’t think her daughter had run away from home.
“It didn’t make sense,” she said.
And Abby Hernandez’s 15th birthday was just three days away.
“She was very enthusiastic about it,” Zenya Hernandez said. “We were planning a whole big party to have friends over to do fun things.”
By 7 p.m. that October day, Zenya Hernandez had filed a missing persons report at the police station. As news of Abby Hernandez’s disappearance reached the community, some wondered whether she had problems in her life or had run away from home.
But Abby Hernandez’s best friend Miranda Cloutier said she didn't see the teen being in a dark place. The last time she saw her in science class, Cloutier said, the two girls were taking selfies.
“We wanted to be silly together,” Cloutier told “20/20.”
Within 48 hours of Abby’s disappearance, the FBI became involved in the case, with agents going door to door in the small New Hampshire town as part of their investigation.
Three days after Abby Hernandez’s disappearance, her birthday arrived. Instead of a celebration, there was a vigil.
Zenya Hernandez shared a message for her daughter in the media: “We miss you so badly, and Sarah and I (are) thinking about you all the time. And we want you back with us, please.”
Investigators were desperate for information. All they knew was that Abby Hernandez had been last seen walking home from school, while her boyfriend Jimmy Campbell was on the school bus texting her. She sent him a heart at 2:53 p.m.
Fourteen minutes later, at 3:07 p.m., Abby Hernandez’s cell signal disappeared about a mile from her house.
“We looked very significantly at her communications with her boyfriend, what her relationship was. We picked apart the relationship they had. We looked at those people who were closest to her,” New Hampshire State Police Capt. Joseph Ebert, who was a lead investigator in the case, told “20/20.”
"We spent weeks if not months of looking at bank accounts and looking at people's locations, looking at alibis to really come to a point of comfort that the individuals that were in her inner circle were not involved in this crime," he added.
Ebert said authorities learned that Abby Hernandez was very family-oriented and close with her mom.
“We just didn’t see anything that was suggestive to us that something she had done had caused her disappearance,” Ebert said.
“I think the things that really started to cause us concern were when we looked at the photograph of her leaving the high school, the video of her leaving the high school, it didn't appear as though she had belongings for a significant period of time away from home,” Ebert said.
After a month had passed, Zenya Hernandez said she was having trouble holding it together.
“I just drove and that was soul-sucking, soul-wrenching. I suspected everyone,” she said. “It’s like one of the things about tragedies like that is that you’re surprised the sun gets up in the morning, and life goes on. I fell into despair."
Zenya Hernandez, so consumed with the ongoing search, ignored routine daily tasks like picking up her mail, which she neglected to do for weeks.
“I was just always looking for her. I did see a girl who looked a lot like her and got out of my car, and I followed her on foot. It was kind of weird, but I did that,” she said.
When she finally got to her mailbox, a letter from Abby Hernandez was waiting inside.
“I can’t believe it and I’m happy beyond belief. And, I’m also confused by some of the language,” Zenya Hernandez said of the letter. “She said, ‘Dear Mom, I miss you and love you more than you can imagine. I’m sorry I did this. I’ve seen the newspaper and TV reports, and to answer your questions, yes I’m alive. ... I miss you Mom, but I won’t tell you where I am.’”
“It’s her, but it’s not her,” Zenya Hernandez said of the message from her daughter.
The letter had been sent 13 days after Abby disappeared and brought renewed hope to investigators.
“We were able to confirm her DNA profile on the letter, but how she came to write that letter, nobody was sure,” Deputy Attorney General Jane Young told “20/20.”
“It sounded like ... the goal of the letter was to say, ‘I’m OK. Now leave me alone.’ It was a real curveball,” Ebert said.
Authorities waited a month to release the letter because they were worried it might put Abby Hernandez in even more jeopardy if made public.
“We had a lot of agonizing conversations,” Young said. “If the victim had somehow been able to sneak a letter out. ... If she was in fact kidnapped, if her captor knew that, she was at greater risk.”
The revelation of a letter from Abby Hernandez caused many in the community to rush to judgment about the case, and a torrent of rumors swirled through the teen's high school, according to Cloutier.
“It was definitely a hot topic for gossip,” she said.
“[It was] devastating, because for us that meant that perhaps they’re never going to look,” Zenya Hernandez said.
Investigators cleared Jimmy Campbell as a suspect, but remained convinced that Abby Hernandez was in danger.
“People just don’t disappear off the face of the Earth, especially a 14-year-old girl. She didn’t drive. We knew that there was money at home she didn’t take. She had only the clothes on her back. No matter where she was, it fit the definition of kidnapping," Young said.
And in a stunning turn of events, nine months after her disappearance, Abby Hernandez walked into her family’s home.
It turned out that she'd been kidnapped. She'd been kept hidden in a storage container against her will and repeatedly sexually assaulted during her captivity. Her captor had her write a letter to her mother that implied she had run away.
Nathaniel Kibby, who'd kidnapped her, pleaded guilty to seven felony counts, including kidnapping and sexual assault. He will spend 45 years to 90 years behind prison bars outside of New Hampshire.
In an exclusive interview with ABC News’ “20/20,” which will air Friday, Sept. 7 at 10 p.m. ET, Abby Hernandez spoke about her experience and what her captor had forced her to endure.
“He said, ‘You know, I'm thinking of finding something a little more humane for you to keep you quiet,’” Abby Hernandez told “20/20.” “He said, ‘I'm thinking of a shock collar.’ You know, that like dogs wear.”
Abby Hernandez said her captor had put a shock collar on her.
“I remember he put it on me. And he told me, ‘OK, try and scream.’ And, I just slowly started to raise my voice. And then, it shocked me. So, he's like, ‘OK, now you know what it feels like,’” she said.
“He told me, ‘Call me, call me Master,’” she said.
Abby and her mother say they are hopeful for the future, despite the traumatic ordeal.
“We want the world to know that horrible, senseless things happen, but there's hope even in the darkest times … and that it's possible to survive even if it's not always joyful,” Zenya Hernandez said.
Watch the full story on ABC News’ “20/20” this FRIDAY at 10 p.m. ET.