Transcript for Girl Testifies At Ex's Trial For Murdering Her New Love
Reporter: As life in the college town of Gainesville rolls on, investigators spent two years building their murder case against Pedro bravo. Erika, burying grief in her studies. She was majoring in biology at Santa Fe community college. During all this time I thought he was near us. Kind of watching over us. And I felt like he was there with us trying to comfort us, trying to give us, you know, it's going to be okay. The truth, the whole truth, and nothing B T theruth. Reporter: But when the trial finally began, the defendant was very much on the defense. High school friend. The trial of Pedro bravo. Reporter: Pedro bravo and Christian Aguilar were high school amigos. But that all ended when Christian swept Pedro's former girlfriend Erika friman off her feet. Very passionate person. Very romantic. I loved that about him. I loved everything about him. I used to tell him, I was like, you know, the world could end right now, and if I'm in your arms, I don't care. Reporter: Now, Pedro stands accused of murdering Christian, and it looks like there's a mountain of evidence to prove it. With all this evidence amassed against him, he's got a very narrow opportunity to tell a story that's even remotely plausible. Reporter: From the outset, the prosecution pulls no punches. Painting bravo as a jealous rival plotting revenge against his former friend. This is a story as old as time, Cain and Abel. Elimination of a rival. Fueled by jealousy, hatred, anger. He has an obsession with her, has an obsession with getting her back. Reporter: Erika takes center stage as the prosecution asks her to recount her story. And now I have to have this person, this, this monster, you know, staring at me, while I'm on the stand. And I can just feel myself, like, aching. And I just don't even want to look that way. I don't want him to look at me. I don't want him to see me ever again. Reporter: The prosecution presents Pedro's journals as proof of his disturbed state of mind. Page after page of teen angst and anguished obsession. Everyone else, wanting me gone, I will give them what they want. Reporter: There was also a self-loathing suicide note he penned in jail. I feel terrible. Every day here is a day spent waiting to die. I am a monster for hurting Chris the way I did. Reporter: At one point, she breaks down in tears. All of this was focused on you. You were at the center of this all. Yes. A lot of it was his obsession for me, and how he wanted me back, and how he wanted to be with me. Reporter: But what did you think, Reading his journal? It sounds like the mind of a sociopath, or a sick person, a very, very mentally -- for someone to be that selfish, and that controlling and that manipulative, you know, it's almost like, when they describe sociopaths, you know, they will kill your dog, and then go help you find it. Reporter: Prosecutors also start piling up the circumstantial evidence compiled over the two-year investigation. The jury learns that just four days before Christian's disappearance, Pedro came here to a nearby Walmart and buys an ankle wrap.)!jq he gets $80 cash back. Then makes his way to this Lowe's. He purchases sleeping AIDS, duct tape and a knife. He pays cash. Then there's that staple of 21 century criminology -- the computer evdience. It turns out that Pedro googled suspicious search terms such as, "Buying anethestics," "What is chloroform?" And, "How do sleeping pills kill you?" At 1:35, he does a search of anesthetic chemical, another search, can rubbing alcohol knock someone out? Reporter: The prosecution claims that days after Pedro completed his homicidal research, he arranged to meet Christian. Reporter Stephanie bechara of ABC affiliate WCJB has been covering the story from day one. The state put together a timeline, a sequence of events where, you know -- where he was, from when he got together with Chris all the way to nighttime. Christian holding that cd, and Pedro next to him. Nobody will ever forget that. Reporter: The two sat parked in Pedro's SUV at that Walmart. According to the prosecution, this was the scene of the gruesome murder. They claim that bravo strangled Aguilar inside his car after knocking him out using Gatorade laced with drugs. Christian Aguilar struggles for his life, he reaches for the door. He scratches the ceiling. This is an enemy burial. This is indifference to life. Reporter: And for the csi types in the jury box, prosecutors have a cornucopia of forensic evidence. The most damning evidence against Pedro is the blood that was found inside his SUV. Reporter: Blood in Pedro's car and on a pair of shoes. Christian's backpack, containing his wallet, was balled up inside bravo's closet. And the residue in his car matched soil samples from the site of Christian's shallow grave. And then a piece of irrefutable evidence. The tear pattern from the roll of duct tape Pedro bought at that Walmart. They literally were able to match the tears of the duct tape found on the victim's body with tears on duct tape on Pedro bravo's own car. Reporter: It is a chilling chain of evidence. But the defense argues that all of it, from the shovel and the computer searches to the poison. Was intended by Pedro to kill himself. He's out there in the woods he's drinking this concoction threw up all over himself and he's like well maybe this is a sign from god that I shouldn't do this. Reporter: Pedro still needed the jury to believe that his statement to police was true. He beat Christian up but left him alive. And now he's going to have to sell that story himself. Coming up, Pedro takes the stand. The jailhouse snitch and the plot to use that shovel to put more victims in the ground.
This transcript has been automatically generated and may not be 100% accurate.