Transcript for Family builds national movement after Vanessa Guillen vanishes from Fort Hood: Part 1
Vanessa was special, like a lot of people tell me, like, I don't know what it was about her that just touches me. Reporter: Vanessa Guillen was the light of her family. She's just a joyful person. She's very outgoing. We just had that connection. Reporter: One of six children, beloved by her siblings and her parents, originally from Mexico, raising their children to pursue the American dream. I always looked up to her since I was little, following in her steps. Reporter: Even from a young age, one thing was clear, Vanessa's sense of duty to her country. Reporter: As soon as she graduated from high school, Vanessa enlisted in the U.S. Army. In her first training, she didn't even want to come back. She was unhappy. Even when she got home she was a new person. Like her skin was full of glowing. Reporter: Vanessa was sent to ft. Hood in Killeen, Texas, one of the army's largest military installations. She worked as a small arms repair soldier, but soon after she arrived, her family said they noticed a change. Reporter: She looks more sad she said. She was not happy at ft. Hood. No. She would try to tell me that she didn't want to be at ft. Hood. I remember this one time she said, "I don't like it here, and I hope one day you understand." On the 22nd, I find that she's not responding at all. No phone calls, they go to voice mail. So I thought, let me message her, maybe the message will go through, and nothing. There's a 4:00 P.M. Accountability check. Vanessa doesn't check in. And that's unusual. So the unit starts looking for her. They told me that her keys, her car, her military I.D. License, credit, debit cards were there. She would never leave her stuff behind. Reporter: After trying to reach Vanessa for hours, her sister Myra takes action, driving three hours from the family's home in Houston to ft. Hood. I thought maybe she had an accident, like we have to look for her. Reporter: She says she arrives in the middle of the night and calls the staff sergeant. She says he tells her to come back in the morning. On the 23rd, the military police were contacted. Reporter: Major general Donna martin is the head of Cid, the U.S. Army's criminal investigation division. They wanted to make sure she wasn't hurt. So when that search came back without any results, then that's when Cid is notified. I texted my sister, like what's going on? She was like, we think Vanessa is missing. I was like, how could she go missing on a military base? That's ridiculous, go find her! And verify her. The meetings at Cid were useless. There was in new information. We gave them literally every single thing they asked for. And for them not to give us an answer, the problem was inside. As a mother myself, there's not enough you could tell me. I would want to know everything. And we can't tell everything. We have to maintain the integrity of the investigation. Reporter: Gloria recalls a conversation she had with her daughter months before she went missing. Reporter: Her mother kept the conversation private at the time. But Vanessa's sisters became the public face of the family. Protesting outside of ft. Hood. We started protesting every Friday. What do we want! Vanessa! Reporter: Inside, the army begins to piece together Vanessa's last-known whereabouts. We know that Vanessa reported to her workstation, which was in the motor pool. And then we know that she came to that arms rail, where she did leave behind her bank card, her I.D. Card and the keys to her barracks room and her car. And then she went to the next arms room, the second arms room, and that is where we believe she came into contact with specialist Robinson. We believe specialist Robinson was the last person to have seen her. Reporter: Specialist Aaron Robinson joined the army in 2008 2018 and was deployed to Iraq. According to investigators, his girlfriend backs up his alibi. Investigators speak to witnesses who say that on the day of Vanessa's disappearance, they saw Robinson coming out of his arms room, pulling a large, So all of these leads lead us back to specialist Robinson. We're interviewing other people in the unit as well, to find out was there a relationship between Vanessa and specialist Robinson that we didn't know about, and there was none. Reporter: Soon, Vanessa's disappearance begins to be reported on spanish-language media where Vanessa's mother Gloria speaks out about the alleged sexual harassment. Family attorney told us she's uncovered at least two incidents of sexual harassment. The military says they've received no credible information about those allegations. The story starts to go viral. The army launches an investigation into the sexual harassment allegations but also trying to handle the criticism regarding the handling of the investigation. Reporter: Vanessa's family keeps up their pressure on the leadership at ft. Hood, demanding answers about the investigation. They finally admitted that foul play was involved. The story explodes into the news cycle. The army suspects foul play now. Reporter: When we come back, Vanessa's family continues their desperate search for their daughter, and we go to the Pentagon to get answers. Mr. Secretary, what in the world is going on at ft. Hood?
This transcript has been automatically generated and may not be 100% accurate.