Other people who knew Martin MacNeill come forward: Part 7

Anna Osborne, another woman he had an affair with, testified about disturbing pillow talk. She claims he boasted to her about killing other people.
6:45 | 06/15/19

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Transcript for Other people who knew Martin MacNeill come forward: Part 7
During the time that Macneill and gypsy Willis are behind bars, it's not just the investigators hard at work. We've been gathering paperwork, whatever we can to help law enforcement. It's Michele's family. They start drumming up interest in her death. If you know someone who may have been a victim, we want to hear from you. We didn't give up. To get the media involved, and "20/20" was huge with keeping that pressure going and keeping this case alive. The victims came out of the Victims of what? Sexual molestation, rape, inappropriateness, as far as a doctor. It became obvious, as this information was coming in, that martin Macneill was a sexual predator. He would intimidate these people. And so they wouldn't file charges. There were no reports written. And he would move to the next hospital. And did it explain this nomadic existence, this sort of -- It did. Why the family bounced around? Absolutely. Do you think that he moved on after some sort of incident of bad behavior or criminal behavior? It appears that way. One of the more intriguing tips came from another woman who claimed to be a former mistress. Her name was Anna Osborne. She described some of the conversations that she would have with martin that were very troubling to her. Anna became just a feast of information. It was during pillow talk with Anna Osborne that martin Macneill talked about previous murders. He bragged to a mistress that he killed his brother, Rufus Roy Macneill because he "Had become an embarrassment." He was high on drugs. And he had a syringe. So he just stuck the syringe in the arm and injected him. He claimed he tried to kill his own mother, and that he had administered several mercy killings at various positions during his medical tenure. He told her he had killed those that were handicapped. In fact, Anna personally believed that he was a serial killer. We requested information on a number of the patients and could never prove anything. She was giving us information that spoke directly to his character. It added a piece of the puzzle, who martin Macneill was. Now with all the evidence that these investigators have managed to compile, they're trying to use it to hopefully pressure the medical investigator to reopen the case into the manner of death of Michele Macneill. He had misrepresented a lot of the facts about how Michele died, where she was, the position of the body and so forth. To distract any suspicion away from him. So a very cold, very calculating person, martin Macneill. He killed Michele, I know he did. We have got tons of evidence. Michelle's sister became increasingly disturbed about the true cause of death and she pushed and pushed and pushed until finally authorities agree to review Michele Macneill's toxicology report and when they did they found a toxic drug cocktail. Oxycodone. Valium. Ambien. Ambien. Lortab. If she is feeling better and doesn't like taking these drugs why would she have these drugs in her stomach on a morning when things are going pretty well? That's a good question. Does that raise suspicions for you? Of course. I would be a very poor investigator if it didn't. We don't know exactly the state she was in when she was drowned and killed. We just know that she was significantly impaired. To the point where she was perhaps passed out, to the point where she was so intoxicated, she would have easily been manipulated. New information raised suspicion. The combination of drugs in her body could have killed her. She may have been forced to take them. Finally, success. It was changed from natural to undetermined. Long story short, undetermined opens the door to the possibility of murder. What is it you think your dad did to your mom? I think he drugged her and then smothered her. I don't know exactly what he did. But I do know he killed her. On Friday Macneill was released from prison after serving three years for fraud. This weekend Macneill returned to the same home where his wife's body was found. He's out. But not for long. Prosecutors are ready to pounce almost instantly. I like to talk about this case as sort of a puzzle. We certainly had some pieces missing, but I believe we had enough, so made the decision to charge the case. They told me that they arrested my father five minutes ago. Martin Macneill is now in a Utah county jail on a $1 million cash bond, facing murder charges. It's horrible to be happy about your father being arrested but finally he's being held accountable for murdering my mother. This is really a bittersweet moment for the Macneill children. Seeing their father shackled here in court is something they've been pushing for for years. This case has no smoking gun. It's all circumstantial evidence. It's all strange behavior and the big question is, will it be enough? Will it be enough to convict martin Macneill of murder?

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

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