Transcript for Colorado man goes to trial for murder of his fiancée
Reporter: She was the mother of a year-old baby girl. New includes in the desperate nationwide search. For the mother who vanished. Without a trace on Thanksgiving. Reporter: Kelsey's mysterious disappearance captivated the country as questions swirled about her fiance, Patrick Frazee's involvement. The next chapter opening in Colorado as the murder trial begins. With every potential juror he locked eyes as if he wanted to communicate with them. Reporter: The main witness? A former lover who could make or break the case. You have a man accused of killing his fiance, the mother of his 1-year-old, in a plot with an ex-girlfriend. I mine, this has all the makings of a TV movie. Reporter: Kelsey met Patrick Frazee in 2016 online. She moved to Colorado to be with him. Kelsey is fun-loving. She's got a bright personality. She is kind, caring. Reporter: She worked as a flight instructor and wanted a family. In 2017 giving birth to their baby girl. She's grounded and responsible. She has a 1-year-old child. She wouldn't just leave her. Reporter: On Thanksgiving day 2018 this surveillance video inside a woodland park safe way captures the last-known images of her shopping with her daughter. But it wasn't until ten days later that her mother, not her fiance, reported her missing. She's not the kind that runs off. Reporter: Once the investigation started questions immediately cropped up. They didn't live together. And Frazee told officers he hadn't seen his fiance since Thanksgiving either when he met to pick up their daughter. If you look at past cases where the husband killed the wife, and you watch their behavior, many of them sort of they're not in obviously, emotionally invested in finding the person because they know they're dead. Reporter: Family and police searched Barrett's home. Inside her makeup, clothes and suitcases left untouched. Kelsey had only lived in this condominium since June. She was quiet and kept to when police came to search they found both of her cars in the driveway and signs that she'd left unexpectedly, including cold cinnamon rolls left on the stove. Reporter: As authorities launched a national search, her mother was there to make an impassioned plea. Call us if you can, we won't stop looking. Reporter: But her fiance was a no-show. Why is he not here? That is a question you'd have to ask him. Reporter: I tried to speak with Frazee first at home. And again when he and his daughter left a meeting at the office of a Colorado Springs defense attorney. Why don't you talk to us? Everybody in their right mind knows that if you lose someone you love you're out there looking. You're lying on the front steps of the police station, begging for justice. What did he do? Clam up and speak through his lawyer! Hm-mm. Reporter: Then, nearly three weeks after Kelsey's disappearance, Frazee's property is searched. Dozens of investigators rolling up to his front gate with dogs, digging equipment and FBI evidence teams scouring the property for two days. You don't take a backhoe to somebody's ranch and start digging if you don't suspect you're going to find something. Reporter: Authorities towed Frazee's truck away. They also discovered what appeared to be a fragment of a human tooth. But they didn't find Kelsey or her body. An explosive moment caught on camera. Law enforcement, guns drawn, pointed at Patrick Frazee. You see him, he's in cuffs. We're just outside Patrick Frazee's property there. And right up there, investigators have Patrick Frazee in custody. They rolled in here just a few minutes ago with an arrest warrant. Reporter: Frazee is now facing eight charges, including first degree murder and solicitation to commit first degree murder, all of which he's pleaded not guilty to. Last December authorities identified the main crime scene as Barrett's townhouse, leaving with bags of evidence, including traces of blood found in the bathroom and on a fireplace. Police also say they discovered a series of mysterious text messages between Barrett and Frazee's phones three days after she disappeared. What is the text message she sent you on the 25th. Barrett's phone was detected in Idaho. In one Frazee texts, do you need can I do anything? Barrett's replies, no, I'm fine, just warn out. Another says do you even love me? Frazee responds, yes, I do. But authorities say Frazee staged those exchanges. A woman in Idaho is now being investigated. Reporter: That cell phone ping giving police a N lead. Authorities are looking at a 32-year-old nurse from twin falls, Idaho. That mystery nurse is this woman. She rekindled a romance. As part of a plea deal, she flipped on her former lover. Authorities allege she did more than just move the victim's cell phone. Lee gives the authorities a very specific account of how she was the one who was supposed to kill Kelsey. Reporter: In an affidavit, she told investigators that on at least three separate occasions Frazee asked her to murder Barrett, suggesting she poison her coffee, even giving her a metal pipe to hit her in the back of the head. She claimed Frazee had described to her in detail how he killed Barrett. Patrick kills Kelsey in a horrific manner. With a bat, in the back of the head, as Kelsey is sniffing candles with a blindfold over her eyes. Reporter: Then following Barrett's murder, Lee told investigators that Frazee called her and told her she had a mess to clean up. She admitted she cleaned the home and burned damaging evidence. He told her the body was stored in a black plastic tote and that he burned the body, pouring at least five barrels of gasoline to keep the fire going. We wouldn't know where the murder happened in the apartment even, in the room without her testimony. It's all going to come down to crystal Lee. Do the jurors believe her? Because it is clear that Frazee's attorneys are going to point the finger at her. The big surprise in the courtroom came from the defense. Their theory, which has never been laid out before is that crystal Lee is to blame for Kelsey Barrett's death. Reporter: Lee denies killing Barrett. Cameras are not allowed inside the courtroom, but Carol Mckinley was there for the first day of testimony. The trial started off with the victim's mother taking the stand. She broke down a couple of times, specifically, when she told the jury about the last time she spoke with her daughter on Thanksgiving day 2018. Reporter: In the weeks that follow, a jury will decide Frazee's fate. One man's life on the line. As those who loved Kelsey Barrett hope for justice. For "Nightline," I'm Clayton Sandell, in woodland park, Colorado.
This transcript has been automatically generated and may not be 100% accurate.