Transcript for Teen Admits to Stabbing Best Friend to Death
for coffee. N "20/20"'s unfriended continues. Once again, Ryan smith. Reporter: University high's leading lady Rachel Shoaf is not able to keep up the act any more. Six months after the disappearance of Skylar neese Rachel's abandoning the role of heartbroken best friend and going from a psych ward straight into the interrogation hot seat. She was just nervous, she was shaking. Reporter: She's finally about to come clean to police about what really happened to her 16-year-old B.F.F. It's plot twist no one saw coming. The first words out of her mouth were, we stabbed her. Reporter: This was something totally different from what you expected. Of settle down. Okay, "What do you mean, you stabbed her?" Reporter: Rachel tells corporal Ronnie Gaskins she and Shelia began plotting to kill Skylar months earlier during science class. They were joking how they would dispose of a body. Then it got more sinister, we need to figure out how we're going to kill her. They don't know to use guns, so they would use knives to stab her. Reporter: Rachel wanted Skylar dead now because, get this, she was about to leave for church camp. So one midsummer's night she and Sheila gather some critical tools. A shovel, paper towels, bleach, wipes, a change of clothes and kitchen knives. They arrange to meet with Skylar. Remember this grainy surveillance video? Police couldn't make out the car. Turns out it had been Shelia's all along. 12:30 A.M., they arrive at Skylar's house. The knives hidden in their hoodies. Five minutes later, Skylar emerges. They drive out of star city, past blacksville and across state lines to Pennsylvania and into the woods of a town called brave where Skylar is about to face the fight of her young life. They are driving on this road that we're on right now and Skylar is in the back seat having no idea what's about to happen? She has no clue. Reporter: Gaskins takes me back to the murder scene. Man, this is dark and creepy. Just imagine, these girls are out here after midnight. Reporter: And recounts what Rachel confessed. All three girls exit the car. And they walk down this road. Reporter: The girls find a grassy area to sit and smoke marijuana. Skylar gets up and starts walking back to the car to get a lighter. That's when they execute their secret plan. Sheila and Rachel get up behind her and count -- one, two, three. And on three, that was the time for them to start stabbing Skylar. Reporter: Right here? Skylar is being stabbed by her two best friends? Yes. Reporter: Skylar takes off running with her two friends giving chase. Rachel tackles Skylar and the pair continue to repeatedly stab their B.F.F. According to Rachel, Skylar is screaming "Why?" And then they tried slitting her throat because they had read that it would kill her quicker if they cut her jugular. Reporter: The plan was to bury her but the ground was too hard, so instead they drag her body to the side of the road and cover her with branches, rocks and leaves. They don't even bother to really conceal her. They change into clean clothes, they wipe down the car and they left. Like nothing happened. This was calculated, planned out, thoroughly thought of before this night. Reporter: The perfect murder plot, everything planned to the "T." But why? Her only answer to that was, we just didn't like her. What do you mean, you just didn't like her? We just didn't like her. Well, why not? Reporter: Did you ever get the sense with Rachel that she really remorseful about what she did? She seemed to be relieved that the truth has been told, she doesn't have to hold it in anymore, it's not eating at her anymore. Reporter: Shockingly, even though Rachel confessed, Gaskins doesn't arrest her. We had no body, we had no physical evidence. I mean, we had all this circumstantial evidence. But we needed something more concrete. Reporter: And there had been no cracks in Sheila's armor. So to try and nail her that night, police put a wire on her friend Rachel, hoping Sheila incriminates herself. But Sheila doesn't take the bait. Instead, the two killers take this smiling selfie together. Sheila tweeting, finally got to see rach. Then a break, police think they have an ace in the hole linking Sheila to the murder. They find traces of blood in her car. We had to do a DNA confirmation. Reporter: Meanwhile, Rachel continues to give critical details to police. Details Skylar neese's parents so desperately await, like the location of Skylar neese's body. And this area right here is where we're able to finally locate, Skylar's remains. Reporter: She was one, two, three, four, five, six, maybe seven steps away from the road? Back in star city the neese family is holding a vigil on what would have been Skylar's 17th birthday. They are finally told the news they've been dreading. Skylar had been murdered. I was outraged. I was a mess. All these emotions running through my head at once. The search is over. She was our world, and now she's not here. Reporter: The identities of the killers was not revealed. And immediately Sheila eddy takes to Twitter to join the chorus of grief. "Worst day of my life." "The pain is real." And "Rest easy Skyler you will always be my best friend." At the same time she is pretending to mourn online Sheila is brazenly taunting police. Tweeting messages like "Wonder if there's a law and order svu where they don't figure it out." And "We really did go on three" a seemingly blatant reference to the crime. But what Sheila doesn't know is that authorities get their biggest break yet, conclusive evidence linking her to the murder. A DNA match confirms that blood from the trunk of Sheila's car is Skylar's. It's that final piece that really nails Sheila. And that news that I was waiting to her. Reporter: Finally with enough evidence to arrest Sheila, Gaskins closes in and tracks her to the cracker barrel where she is having breakfast with her mother. I just want to be able to get her right then and there. Reporter: It's a race against time at this point. We're not just dealing with a 16-year-old girl. This is a cold-blooded murderer. Reporter: It's the end of road for Sheila. She is leaving the cracker barrel just as Gaskins pulls up, cuffing the teen killer right there in the parking lot. I advised them, she was being charged with the murder of Skylar neese. Reporter: Also a huge victory for that young cop who wouldn't quit, Jessica Colebank. Elation. Pure joy. Reporter: And what did you say when you brought her to jail? Home sweet home. And then we shut the door, with a big old grin on my face. Reporter: Case closed for Colebank, but for Skylar's parents Dave and Mary, learning the killers' identities not only brought more heartache and devastation but also fury. They're acting like they really care and they're the ones that are responsible for the whole thing the whole time. Reporter: Casting devastating new light on that moment when Sheila was overcome with emotion in Skylar's bedroom. Oh, my lord, we believed that garbage. Can you imagine? Your daughter's killer, sitting on your daughter's bed, crying. And the mother, putting her arms around her daughter's killer. Skylar trusted those two little witches and it cost her her life. Best friends like that, you don't want any enemies. For sure. How could they do that to their best friend? Reporter: And the reason Rachel gave -- that they just didn't like her? That's the biggest line of crap I've ever heard in my life. There has to be a reason. You don't do stuff like that for no reason.
This transcript has been automatically generated and may not be 100% accurate.