Was martin mcneill about to get away with murder? That's what two unlikely accusers believe his daughters. They say he killed their mother and they have spent the past six years trying to prove it.... See More
Was martin mcneill about to get away with murder? That's what two unlikely accusers believe his daughters. They say he killed their mother and they have spent the past six years trying to prove it. From a suspicious nanny to an unnecessary surgery, the puzzle pieces they claim are finally in place. My co-anchor dan abrahams brings us the latest compelling testimony for our series, crime and punishment. Do you recognize what that is, rachel? Yes. What is that? It is a picture of my mother. Reporter: It was a dramatic moment in the 5-day-old trial. Rachel mcneill called to testify against her once beloved and admired father martin, for the suspected murder of her mother michelle. Do you recognize this man sitting right here? Yes. Who is he? It's my father. Reporter: Sitting feet away from him in a packed utah courtroom, her voice unsteady, the shaken daughter talked about the man she once loved so much. How close would you describe your relationship with your dad growing up? Very close. Why would you say that? Growing up my father was my best friend. Very close to your dad? I was close to my dad, yes. Reporter: Rachel and alexis have been on a six-year campaign for this moment to see their father, a doctor and prominent member of this utah town tried in connection with the death of their own mother. In fact, without their persistence, this case would likely have never come to trial. Ever since the day my mom died, i, I -- was concerned that my father killed her. And I have been fighting to get justice for this case ever since then. He did not count on his daughters actually turning against him. He did not count on his daughters, putting two and two together to get the answer of four -- murder. They did. And they never gave up. Reporter: Prosecutors believe in 2007, mcneill pressured his wife, a former beauty queen to get a facelift. But with only one real goal in mind. To overmedicate her during her recovery and ultimately kill her. It was my father's voice. Reporter: Rachel recalled the phone call from her father. Rachel, quick, get to the hospital. It's your mother. Quick. Reporter: Mcneill and his lawyers insist michelle died of natural causes. But this week, a parade of prosecution witnesses raised questions about mcneill's seemingly bizarre behavior the day of michelle's death. Including a 911 call where he claimed he needed help to remove his wife from the couple's bathtub. Mcneill sent his 6-year-old daughter ada over to the neighbors christie and doug daniels. My dad needed help. He needed a male's help? That's what I remember. Christie called her husband who came right over. I could have got her out myself. Reporter: A key point for prosecutors trying to show mcneill was not trying to save his wife but trying to delay help from arriving. Defense attorneys dispute that. Arguing that martin mcneill had an injury and immediately did everything he could to save his wife. And the cops on the scene believed it was an accident. The medical examiner, listed the cause of death as natural. But there was more. They didn't know at the time. Twice he would say -- why? Why would you do this? All because of a stupid surgery. Earlier the plastic surgeon who performed michelle's facelift testified it was martin, not michelle, who seemed far more intent on the procedure. So much so that mcneill was even requesting the doctor prescribe medications beyond what was his usual practice. Why did you prescribe these extra drugs? That was because martin indicated to me that he was very concerned about his wife that she didn't handle pain well. Reporter: Police never found the drugs because mcneill asked his son's girlfriend to throw them out shortly after mid s michelle's death. Did he ask you to flush them down the toilet? Did that seem strange to you? At the time, yes. Reporter: Something else that seemed strange, within weeks of michelle's death. Martin hired some one to care for his younger daughters. But gypsy willis was no super nanny. She didn't to anything that i had seen related to the children. What did you see her doing? My dad was cooking. She was sitting there, staring at my dad. Things look thalike that. The children were taking care of themselves. Before her death, martin suspected that jipsy was having an affair. When alexis objected to the family's new care giver. Martin took action. He said that my mother had thought that he was having an affair with this woman. And that that wasridiculous. And that alexis was going to be that way she was no longer a member of the family. Reporter: One of the children gypsy was to care for, 6-year-old ada plays a crucial role in the case. After all it was the little girl who first discovered her mother's body. Prosecutors argue, that was all part of martin mcneill's plan. He physically showed step by step how he found my mother and talked about ada first finding my mother. Reporter: Ada's testimony may poke a hole in mcneill's defense. Prosecutors say ada recalled the crime scene, very differently from martin mcneill. He specifically said to me that he was concerned that there be police investigation. That he didn't want to, any one to think that he murdered my mother. It was so shocking to me. I said, why, why would anybody think that? Why would any one think that? You never said anything to doug whitney about your dad joking about being single. Reporter: On cross-examination, mcneill's lawyer questioned rachel's memory. Suggesting she changed her story to make it more incriminating. All right. Rancheros and even asked about her history of mental illness. You have been diagnosed bipolar? Have I in my lifetime been diagnosed as bipolar? Yes. The defense insists mcneill has an alibi that he couldn't have killed his wife they say because he was at a work event at the time. But prosecutors don't buy it. Arguing the supposed alibi was just a part of martin's sinister plan. At the event, colleagues say he acted strangely. He told me he needed his picture taken. He was adamant. Reporter: In the coming days daughter alexis is expected to be called back to the stand. And tomorrow, that infamous nanny, gypsy willis, is scheduled to tell her side of the story. Court is in recess. For "nightline," dan a brahms, new york.
This transcript has been automatically generated and may not be 100% accurate.