Transcript for Aaron Robinson’s girlfriend allegedly confesses to helping dispose of body: Part 7
After finding human remains out by the Leon river, the army detains specialist Aaron Robinson. They're monitoring him, but they don't want to tell him the real reason why. At about the same, investigators track down his girlfriend, Cecily Aguilar, and speak with her again. According to them, Aguillar changes her story about where she was the night Vanessa disappeared. Now instead of spending the night at home looking for a lost cell phone, she allegedly tells them she and Robinson took a late night drive to go star-gazing. But investigators don't buy Cecily's new alibi for Robinson. According to court filings, one of them even says to her, I know you're lying to me, again. And they say eventually she changes her story again. Cecily Aguilar did tell us that specialist Robinson told her that he killed Vanessa. As Aguilar's interview with investigators continues, she tells them that Aaron Robinson confessed to her that he had bludgeoned a female soldier to death in an armory room on base and then placed the remains inside a plastic tough box. The arms room was not thoroughly processed until her remains were found. How did that impact the investigation? If you'd gone into the arms room and just shined some pluminol around the arms room, there would have been a spectacular showing of blood. And that alone should have focused them in that one place, that one person. Investigators say Aaron Robinson smuggles the plastic tough box containing Vanessa's body into his car and then drives off ft. Hood. Aguilar tells investigators that she's at work at a gas station on the night that Vanessa disappeared and that Robinson drives to the gas station, picks her up. According to a criminal complaint that details what investigators believe happened to Vanessa, they then both drive out to the Leon river in the dead of night, and Aguliar agrees to help Robinson get rid of Vanessa's body. They allegedly decide to use some type of tool, reportedly a machete of sorts, to then dismember the body. Then investigators say that Aguilar told them they attempted to burn the body, but that didn't work. So according to the complaint, they then dig three separate holes to bury Vanessa's remains. They cover them up and they leave. But then three days later on April 26th, investigators say they come back again bringing hair nets, gloves and concrete. They allegedly go to the remains, attempt to further break up the body, burn it again, and then decide to cover up the body with concrete. They clearly come back, according to this criminal complaint, with a plan on how they are going to conceal this body as best as they can. That really goes to the personality of that offender. Robinson has taken this young woman's life. And now most people would be in such an emotional state if they had done that, but not in this case. He's now able to start thinking strategically. What do I do with the body? That's not typical, no matter what you see on television or in the movies, that's not typical. Juan Cruz learned the gruesome details about his fiancee's death from Twitter. The details are so disturbing that her family decides not to tell Vanessa's mother exactly what happened to her daughter. It hurts me knowing that I know the truth, but she doesn't. As a mother, she's supposed to know, not me. But she doesn't know. And so it hurts me the most, because I shouldn't be a liar. I shouldn't be telling her, no, no, that's not how it happened. But I mean, one thing we have to tell her -- The details alleged in that complaint about what happened to Vanessa are so horrific that Cecily Aguilar's friend ayrren can't even wrap her mind around them. I just can't see my best friend doing that to another human. For her to do this with a man she barely knew, it just doesn't -- it doesn't make sense to me. People do things out of character all the time. And those closest to them will say, after the fact, I don't believe it. I just never saw that. They're not capable of that. We would hear that all the time in the FBI. So that night whi investigators say Cecily Aguilar is making a confession to them, Aaron Robinson is about to make a run for it. How does this guy who's being monitored escape? He's somewhere in Killeen, Texas. And Robinson does something that I don't think anybody saw coming.
This transcript has been automatically generated and may not be 100% accurate.