Daughter Says She Helped Mom Try to Kill Dad, Siblings: Part 4

Rachel Staudte told police they chose anti-freeze because 'you could put it in something and you couldn't taste it.'
8:40 | 01/29/17

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Transcript for Daughter Says She Helped Mom Try to Kill Dad, Siblings: Part 4
Reporter: As the freight trains roll through Springfield, Missouri, the wheels of justice are turning slowly too. Police are interviewing 22-year-old Rachel staudte. A star student, a gifted musician, and artist. Mom's pride and joy. Mom was there every step of the way really encouraging and cheerleading Rachel to be great. Reporter: But now, her dearest mommy, Diane staudte, is behind bars after confessing to the poisoning deaths of her husband and son and the attempted murder of Rachel's older sister. Rachel, can I get you to switch with me? Okay. Reporter: Police just hit Rachel with the news. Confusion. Shock. I mean, that's, nobody expects to have somebody that they know do that. Reporter: In this interrogation tape obtained by "20/20," detective Mcamis lays it all out. Demure Diane has delicately engineered a ruthless culling of her own herd. Only the pious and the productive will make the cut. Did you think somebody else was going to be next? Sarah can be poisoned just for sitting around on youtube, I don't do anything around the house. Brianna sure as heck doesn't do anything around the house. Reporter: Rachel fesses up that her mom had some unusual Reading material. We had a book on poisonous plants. She would talk about cyanide. Reporter: Then a crucial -- critical moment across town. These pictures show a house in disarray. This was the home, the second residence. Detective Mcamis gave us a tour inside with permission from the new homeowner. This was Sarah's room, there was couches, papers, kind of things streamed about. Reporter: Laptops, flash drives, sculptures, scattered about. And in the garage -- paydirt. This is where our crime scene detectives located the antifreeze on the work bench. Reporter: And right next to the antifreeze, the other compound in Diane's cruel chemistry -- an otherwise unassuming six-pack of coke. Coke was described as what was given to both Shaun and Sarah in terms of poisoning them. Reporter: The concoction right there? Yes. Correct. Reporter: The coke and the antifreeze. That's it. Reporter: In the bedroom, a curious detective makes that dynamite discovery. This is Rachel's purple diary, haphazardly thrown on a shelf. Its contents, a trove of sinister secrets. It appeared to be in a journal entry written by Rachel, knowing that mark and Shaun were getting ready to be killed. Reporter: So at first, you're thinking this is a mother, an evil mother who's tried to wipe out her family. And now you discover her daughter might be involved helping her? Yes. Reporter: That journal entry, dated June 13th, 2011. Nearly a year before mark died. It reads -- It's sad when I realized how my father will pass on in the next two months. Shaun, my brother will move on shortly after. It will be tough getting used to the changes but everything will work out. Reporter: That's pretty chilling. Very. Reporter: She's writing in a journal about killing her father and her brother. It was extremely alarming. Reporter: Back at the station, detective Mcamis leaves the room and gets briefed on that explosive diary. He returns in a different mood. Rachel, do you recognize this? Yeah, I remember this. Reporter: This is Missouri. And now it's time to show me. You wrote this then, is what you're telling me? I had a lot of really bad dreams about them dying. I talked to mom about it and she mentioned she was thinking of hurting them. Reporter: Rachel's bad dream, just beginning. Mcamis isn't buying her story or her tears. What did you tell your mother? That it would be quick. That it'd be easy. Reporter: Rachel reveals the awful truth. Mother's favorite was also mother's little helper. When did you guys come up with this plan? We talked about it like Christmas. Reporter: She's sitting right here. She started to tell the whole thing. She basically owns up to it. To hear the daughter and the admissions that she's making that this was a plan. I was totally shocked, totally stunned by everything that had taken place. It was just a completely surreal feeling. Reporter: The mother-daughter duo carefully plan and research all options. Suffocation, pills, googling how to kill your husband. The devout Christians even explored witchcraft. On my computer specifically there's a lot of wicca sites. Reporter: Before settling on antifreeze. Because in general you could put it in something and you couldn't taste it. What else did she tell you? She wanted a specific tasteless one. Rachel, whose idea was this? Mom brought it up and then we discussed. Mom recruiting daughter to help her systematically kill her husband, her son, and then poison her daughter? It's almost unheard of. Reporter: It was supposed to be just dad, but pretty soon Diane turned her attention to Shaun, the son she thought was irritating. Listen to how little it took to end up on her hit list. Shaun, we argued on a lot 'cause I still think we could have put him in an assisted living, but she wanted him out. What did you say when she talked about killing Sarah? Sarah was equally unneeded. We could have found someplace else for her. She was very adamant on that Sarah was just a burden, that Sarah needed to be taken care of. Reporter: Rachel, too, now under arrest, charged with murdering her father and brother. Her sister, Sarah, still fighting for her life. When Rachel was arrested, there was a big clue in her purse. Her purse was searched, incident to arrest. And in there was a note. Reporter: With her sister Sarah hospitalized, Rachel had authored a poem evocative of Edgar Allen Poe. Once upon a time there were six. Now there are three. Only the quiet ones will be left, my mom, my little sister and me. Reporter: Wow. That's pretty cold stuff. This is very cold and depraved murder. The Springfield poisoning case. Two people dead, two in jail. One in the hospital, the family was poisoned with antifreeze. Reporter: The next day, the tale hits the news like a twister. While down at the county courthouse, a rare sight. A mother and daughter, both arraigned for murder. No, way. This cannot be true. Not right here in our little neighborhood. Reporter: But no one is more surprised than mark's old bandmate, Charles Alexander. I was floored. When I heard that he was poisoned, then I just cried, I collapsed. I said, "I can't believe this woman did this." Reporter: Looking back, he begins recalling little quirks about his friend's wife. I was never really allowed in the house, for some strange reason. It was always at the garage. It was so strange, I seen antifreeze bottles sitting in the garage, I'm thinking, who has antifreeze in the summertime? Reporter: Strangest of all, Diane's blase manner after mark died. I went to her house, and asked her what happened. And she proceeded to tell me, like she was giving me a recipe to a cake. No emotions, no nothing. Reporter: Just matter of fact. It was like, oh, he died, and you add two eggs. Reporter: Also shocked, Diane's fellow church members at redeemer lutheran. Well, maybe except for one. Remember that anonymous 911 caller who first tipped off police? Do you ever find out who blew the whistle on her? We were able to determine that the anonymous caller was actually the pastor at Diane's church. Reporter: The pastor turned her in. Yes. Reporter: Pastor Jeff Sippy. If you are not praying for your children, no one else is either. Reporter: The leader of the flock, compelled by his own conscience to root out the wolf among them. He declined our requests for an interview. So he knew that something was evil here. I think he definitely had his suspicions. Diane's demeanor after the deaths. He told me couldn't take those feelings that he had to -- he had to call it in. Reporter: When we come back,

This transcript has been automatically generated and may not be 100% accurate.

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