Tonight, the heartbreaking and horrifying new evidence in a case we've been following for months. We're hearing the interrogation tapes for the first time, 12-year-old girls describing why they... See More
Tonight, the heartbreaking and horrifying new evidence in a case we've been following for months. We're hearing the interrogation tapes for the first time, 12-year-old girls describing why they stabbed their friend 19 times and left her A brave journey, and no one could imagine what we heard in these interrogations heard just this week. A community hearing why they did this for the first time. Reporter: Tonight, like much of this country, it is a snowy and frigid Friday night in one American town, waukesha, Wisconsin. Where this evening, an entire community is stunned all over again after hearing this week for the first time from two girls, their own voices, about the night they allegedly lured their friend into the woods and repeatedly stabbed her, police say, leaving her to die. She handed me the knife and told me to stab her. Reporter: "20/20" has been following the case every step of the way. It's a horror story that began more than eight months ago. A mother and father like so many parents across this country sending their daughter on a sleepover. At the time, Payton leutner was just 12 and her parents knew her friend well, Morgan geyser, and another 12-year-old, all of them celebrating Morgan's birthday. Why would this night be any different? But it was. On Saturday, may 31, 2014, a 12-year-old girl was stabbed. Reporter: Tonight, the explosive new interrogation tapes just released. What are you doing? Reporter: As we take you back inside a case that horrified parents across this country. Two 12-year-old girls charged with viciously attacking their friend. They stabbed their friend 19 times. Reporter: Those girls, the suspects, in their own words in the hours after they were taken into custody. It is the first time we hear from Morgan geyser talking about her friend Payton, who they left in the woods. She was my only friend for a long time. Why would you hurt your only friend? It was necessary. Reporter: And for the first time we also hear from Anissa Weier, too. Her face blurred, the judge releasing this footage with the requirement not to show their faces during the interrogations. I said I'm scared. I was told, I was told if I didn't do something, my family would be in danger. Reporter: Her family in danger? Who and what were those girls so scared of? We explore that right here tonight. While parents everywhere might be saying, not my child, tonight you're about to hear how all of this began as an ordinary slumber party. Three girls and someone else, an unknown figure getting into that party, too. A fictional character from the internet named slenderman, who so many parents had never even heard of. We have spent months in that town and have documented one family's journey. Two parents helping their daughter recover. She was forced to go into the woods. Reporter: Into the woods. From the hero who found her at the edge of the woods. She was laying right here. Reporter: The doctors who saw how close to death she came. The knife cut through the tissue. Reporter: And the school where Payton has now returned. One family's stunning journey about to play out tonight right before your ey. And just this week, two parents who heard for the first time from the friends who police say attacked their daughter. It was a Friday night, right? It was a Friday night. Reporter: Their daughter Payton had been looking forward to it for weeks. You remember how excited she was that Friday afternoon? Oh my gosh, she was so, so excited. Reporter: And had she gone on sleepovers before? Oh, yeah. This was just -- you know, her best friend's birthday party. Just another -- it was just another Friday night. Reporter: Have some pizza, and spend the night at her best friend's house? Yep. They were probably going to watch a movie. And play American girl. Payton brought her American girl doll with her. Reporter: But Payton would not come home the next morning. Do you think Payton had any idea? No. She had absolutely no idea. She was blindsided. Reporter: Her parents blindsided too. Friday night they send their daughter off. Saturday morning, mom is in the backyard. Joe was at work. And I heard the doorbell ring and the dog bark. And it was a Saturday morning at, like, 9:30, 9:45. And around the side of the house, up over the deck came a uniformed officer. The first thing that goes through my mind is, something has happened to somebody that I love. Because that's the only reason a uniformed officer and a detective comes to your house on a Saturday morning. Reporter: She would slowly begin to piece together what had happened in the 16 hours since Payton had left for that sleepover. It is unthinkable. The first moment anyone would begin to learn of that horror was this call to 911. 911. I'm transferring over a caller on big bend. Reporter: After she'd crawled out of the woods. And just listen. At first the operators cannot believe what they were hearing. He came upon a 12-year-old female. She appears to be stabbed. She appears to be what? Stabbed. Stabbed? Correct. Reporter: Greg Steinberg was riding his bike that morning. On a path that had actually been chained off. It was pure chance that he came this way. Sir, are you with her right now? Yes. Is she awake? She's awake. Is there any bleeding going on? Her clothing has got blood on it. Okay, and you found her and she was just laying there? Yeah. Reporter: You can actually hear him comforting Payton. Honey, he's coming. They'll be here any minute. Who did that to you? Reporter: And you were biking by, and she says to you what? "Could you help me please, I've been stabbed multiple times." I quick got out my cell phone I was shaking, and dialed 911. I just stayed with the 911 operator and I offered her water, cause I had a water bottle on my bike. The other thing she said was she was having trouble breathing. Reporter: She was right here lying on the grass. He watches as the ambulance rushes her away. The hospital less than 4 miles away from that path. And when you looked at her? It was immediately apparent she'd been stabbed multiple times? Yeah. To her chest, abdomen, arm and leg. Reporter: Doctors concerned she might not survive. And her mother Stacie has just been told Payton has been rushed to the hospital. She's also been told it was her best friend Morgan who did this. She calls her husband. And I said, "Payton's been stabbed. Morgan stabbed Payton. You need to come home." They need to rush Payton into surgery. So then I really lost it. Reporter: So you got in the car? Got in the car. Reporter: Minutes later arriving. So I walked into the trauma room that she was in. And she was pale as a ghost. She was terrified. She was crying. She couldn't breathe. Reporter: But she saw you there? She saw me. And she put her hand out. And I rushed over to her. And I put my arms around her. And I laid next to her. And I hugged her. And I said, "You're going to be okay. It's going to be fine." But I could see that she was covered. Her arms and her legs and her abdomen, they were covered in stab wounds. Reporter: They pull her away from her daughter. And she can soon hear the nurses counting. And all I hear is, "There's five on her arm. There's seven on her leg." And I'm thinking, "What? Seven? What?" And one of the nurses says, "All right, I count 19." And then the second nurse said, "I count 19, as well." 19? What? There's no way. Reporter: There were so many stab wounds, it took two nurses to count them, 19 in all, and her little girl is now being raced down the hall. Then they took her into surgery? Then they take her into surgery. Reporter: Did you say anything to Payton as they were Wheeling her away? That I loved her and that she would be okay. Reporter: But there was something else Payton's mother was thinking at the same time. Morgan didn't do this is what's going through my head. There's no way. There no way that's what's happened. Morgan is 12. Reporter: But that's exactly what investigators believe, that Morgan, and the only other girl who was invited to that sleep over, Anissa Weier, did this. But they were nowhere to be found. They had run away. And -- and the police hadn't found them yet. And we just knew that Payton told us Morgan stabbed her. And the police hadn't been able to find the girls. Reporter: And in fact, police believe those girls were walking. Yep. Reporter: And tonight, right here, we finally hear the detectives, the investigators, who searched for those girls, who found them. Detective Michelle trussoni on the stand just this week. Once they left that wooded area they started walking. They could hear the sirens coming. Reporter: They were going to find a mansion in the woods? Oh, the mansion, yeah, the mansion in the woods. They were into the Nicolet forest, because they believed that there was a mansion there that slenderman lived in. Reporter: This week, we heard the interrogation. Just listen to Morgan. In her own words. Acknowledging who they were looking for: We tried to find slender man. Tried to find who? Slenderman. Who's slenderman? He's a this tall, faceless man who preys on children. Reporter: A character named slenderman. Who police say they were hoping to impress. All while their 12-year-old friend Payton is in the O.R. Fighting off death. When we come back -- the doctors who discover, it is worse than anyone thought. We're with the surgeon who saves her. Less than a millimeter. Reporter: Between living and dying. Yes. Reporter: Tonight, you'll hear much more from them in their own words, the hours of interrogation tapes just revealed. Why did they do this? What was discovered on their computers, their cell phones? And later tonight right here, our time with the survivor Payton and a brand-new milestone one week ago tonight. A young girl you will never forget. Fibromyalgia,
This transcript has been automatically generated and may not be 100% accurate.