Reporter: Let's face it. We live in the United States of surveillance where every moment -- from the shocking and the trivial to the outrageous and mundane is an open target for exposure. And on... See More
Reporter: Let's face it. We live in the United States of surveillance where every moment -- from the shocking and the trivial to the outrageous and mundane is an open target for exposure. And on September 20th, 2012, almost every moment of Pedro bravo's day was captured in the bitter grays of surveillance cams. He and Christian meet at 1:39 P.M., head to lunch then go to this Best Buy. A receipt shows Christian bought a Kanye west cd with cash and used his rewards card. At 4:11 that afternoon, bravo drives his friend to this parking lot. The two sit in bravo's Navy blue blazer at this spot. No one gets in or out of the vehicle. About two and a half hours later, security cameras show bravo pulling out of that Walmart parking lot. Christian Aguilar is never seen alive again. The last person that was with your son was your son's girlfriend's ex boyfriend, we all started having suspicions. Reporter: Now, at the same time that Pedro was being tagged by all that surveillance, Erika friman was frantically trying to find Christian. I kept texting him and kept calling him and it was really strange that he wasn't answering. Was -- wasn't like him. You know, I'm trying to calm myself down, you know. Yeah, yeah, there's a perfectly reasonable explanation for this. Reporter: Back at Pedro's apartment, what security cameras pick up next was suspicious and disturbing. Bravo arrives then immediately leaves his apartment. Where he goes next is an issue of contention because 30 minutes into his drive, he deliberately turns his phone to "Airplane mode." He puts it in airplane mode where his phone isn't gonna be detected. Reporter: Ken Brennan is a private investigator hired by "20/20" to review the case. He's sharp enough to know that the police can try Ang late cell phones. Reporter: He's off the grid for almost five hours before surveillance cams pick him up at a nearby McDonald's, having a big Mac and a Dr. Pepper before finally heading home. He knows that I'm gonna dump the body and I wanna dump it far enough away from, where the crime occurred. So, he wants to get the body out of Gainesville, far enough away where it's in another jurisdiction. It's a lot harder for the police to investigate a crime that's outside their jurisdiction than one in their own. Reporter: 4:00 A.M. In the morning and Christian is still missing. I call Pedro, and it starts ringing. And it's ringing, it's ringing and then finally he answers. And you know, the first thing I ask him was like, "Where's Christian?" And he's just like, no, you know, I dropped him off, you know, we got into a verbal argument, you know. I'm freaking out. And he's just like, no, listen, you know, it's late, I'm sure he made it home, he just didn't call you. Reporter: By daybreak, Erica is panicking. You call his apartment, he's not there. He's not at your place, either. I call Pedro, and at this time, I'm not asking him, I'm telling him, like, listen, you need to come with me to the police station. Reporter: 9:44 in the morning and both Erika friman and Pedro bravo go to the campus police station to report their friend is missing. Sergeant Steve wilder was suspicious from the get-go. When you first saw Pedro bravo come in there, into that room, what was your first impression? It didn't take long, interacting with him and talking to him, that I knew that there was really something that he was holding back. I don't know where he is. Reporter: At what point do you begin being suspicious of Pedro? Honestly, we even walked into the police station. I still didn't think Pedro was capable of doing something so horrendous. I mean, this guy that's, you know, shy, and, you know, he wouldn't hurt a fly, you know, he loves animals, and you know, he's hard working in school. Reporter: Never seemed violent. Never seemed violent. Reporter: Pedro says he came here to help, but everything that comes out of his mouth only cements his status as the prime suspect. We're finding out new stuff that you haven't talked to me about before. Reporter: At first, Pedro tells investigators that he left Aguilar in the woods not far from Walmart after arguing with him in the car. I told you everything I know. I just drove away, like, while he was still there. Reporter: But with every question, a new variation in his story. Now, Pedro adds that there was some kind of physical altercation. Okay, so you strike one time, blood or no blood? He was already bleeding. So, you strike him with your other hand, right? Yes, right here on the cheek. It had gone from, well, I punched him one time and made him get out of the car, to I forced him out of the car, punched him repeatedly to I punched him repeatedly, landed on top of him, punched him again repeatedly and left him unconscious but still breathing. I'm gonna be honest. I wanted to hurt him. Lie lierk, like, really hurt him. You can see, he realizes I'm with a guy that knows what's up. I might be able to B.S. Some people, but this guy knows better. Reporter: Was there a time when you said, I'm speaking to a murderer? I knew I was speaking to someone that was being extremely deceptive. Reporter: You saw Pedro bravo. He's not a very big guy, and when you think of him knocking somebody who's eight inches taller than him out of the car and beating him up, what do you think? They have that old saying, "It's not the size of the dog in the fight, it's the size of the fight in the dog." I turned around and just punched him in the face. What did he do? He grabbed his nose because I punched him. I turned around and hit him in the face. When he starts fumbling over himself and he starts coming up with these Stories, they know they got the right guy. Now, it's a matter of gathering the evidence enough to be able to arrest this guy and then ultimately to convict him. Reporter: After eight hours of interrogation and rambling answers, Pedro has still not confessed to any crime. But tellingly, he keeps referring to Christian in the past tense. Christian was a really good friend. We know that he's not coming back, okay? You know he's not coming back. Reporter: Look at that again. A subtle but fateful nod of the head. And with that, the case against Pedro bravo had begun. And so had the search for Christian Aguilar's body. You have the right to remain silent. Reporter: It was the largest and most publicized search in Gainesville history. Hundreds of people -- friends, family, volunteers, and even psychics combing ten square miles of woods and ditches in search of clues. I'm looking for people's heart that can come and empower me. I'm looking for somebody that can walk with me. That's the only thing that I ask.xai; God, today we pray for the Aguilar family. Reporter: Candlelight vigils pop up for the missing teen in Gainesville, Miami, and the family's native Colombia. We had hope of finding him. I mean, it slowly started going away when you're reaching day four and five, and there's no trace of him. So, you knew that he was dead, but you were not willing to accept the fact that he was going to remain missing. Yes. Reporter: Eight days after Christian's disappearance, even though no body has been discovered, the state charges bravo with kidnapping and first degree murder. It was sickening almost, just because, you know, we -- we knew him for so long. And, you know, Christian was his friend. I'm just there like, you know, crying, crying. And we're just feeling like, you know, he's -- he's not going to come back. We're not gonna -- we're not gonna ever see him again. Reporter: Knowing that Christian's family and the love of his life are all in agony, Pedro still offers no information about his friend's whereabouts. Then, 22 days after his mysterious disappearance, two hunters in search of firewood found Christian's body in the swamplands 60 miles southwest of Gainesville. The family tells us they feel like it's their son. And they tell me, you know. Listen, the search is over. You know, they -- they found him. Christian's -- Christian's gone. And I mean, now I'm starting to shake, and it was like, this sense of -- of relief and at the same time, this sense of overwhelming pain. Reporter: When we come back, lights, camera and courtroom action. To prove how the young freshman taught himself murder 101. As the suspicion and evidence mounts tonight, our question, have you ever been involved with someone that can't let go? Use #2020. We'll be right back.
This transcript has been automatically generated and may not be 100% accurate.