Investigators Discover Martin MacNeill's Life of Lies

Act 4: Two years after Michele MacNeill's death, the case is reopened.
3:00 | 02/21/14

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Transcript for Investigators Discover Martin MacNeill's Life of Lies
Once again, Elizabeth vargas. Four years ago, two desperate sisters took their story to "20/20." After a small-town police department closes the case on their mother's death and let their father off scot-free. Not only would they not listen, they were mocking us, as well. They were saying, "You're ridiculous, you guys are just upset that, you know, your dad has had an affair. Lots of people have affairs." Reporter: But they found a sympathetic ear in investigator Doug witney at the Utah county attorney's office. Two years after Michele's death they convince him to re-open the case. And when he does, he finds something surprising. Martin was not the man he claimed to be. This prominent doctor and lawyer, actually had a life based on three decades of lies. Stretching all of the way back to his days in college and his school transcript. His entire career is based on falsified transcripts from different colleges. Reporter: So how did he go about practicing medicine if he hadn't really gone to these colleges? Well, the guy is brilliant. I am not saying that he is not smart, he just -- Reporter: And that was only the beginning of his lies. The summer after Michele's death, they apply for a new social security card using her personal information and gypsy's name. They went into court and changed the birth date 20 years. That's called perjury. Now there is two people with that social security number, two people with that name. Reporter: That was only the beginning. How did this idea come up to take Giselle's identity, her social security number, her passport, and adopt it as yours? When I got together with martin, I had a lot of tax debt. Probably in the range of $50,000 or $60,000. This was martin. This was martin's idea, this was martin's activity. Reporter: So why did you do it? I said I didn't want to. He said that this is the best way to do it, it's temporary, Reporter: But someone did namely, witney, when he started looking closely at Michele's death. They discover that martin and gypsy are amassing fake ids all over Utah. You are talking about a state I.D. Card. You are talking about bank accounts that were opened up under false names, false social security Numbers. Martin did a lot of things that were, um, not very sensible. Reporter: So you're saying martin did all of this without your knowledge? He would tell me he was doing Thi things. Reporter: You got two fake military I.D. Cards, three joint banking accounts using a fake name, you got a fake birth certificate, a fake Utah state I.D. Card, and a fake social security card. How could you be a passive participant in that? Because martin was very powerful. He was very influential. I just assumed that he knew what he was doing. He'd been successful all of his life. Reporter: Gypsy, you're an educated person. You bonded over quantum physics. And now you want me and us the authorities to believe that you just checked your brain at the door and agreed to all this? When you love someone you do things that don't make a lot of sense. He had everyone confused. He was this pillar of the community. He'd been bishop twice. I mean, he had all of the accoutrements to be convincing. Reporter: His gigantic con job was stopped short by Doug witney's investigation. He had both martin and gypsy arrested for identity theft and they were convicted in federal court and sent to prisons in Texas. And while they are behind bars, investigators go for broke, and ask to pin Michele's death on martin. I will simply say that I believe that martin Macneill is a sociopath. Reporter: They ask some tough questions. Why couldn't a fitness addict like martin lift his wife out of the bathtub? Could martin's relationship with gypsy be a motive for murder? The questions raise suspicion. Then, prosecutors catch a break and weeks after his release on fraud gechars, he's arrested again, this time for murder. A big trial getting under way in Utah. No doubt in my mind that one day this case was going to be re-opened. Reporter: 5 1/2 years after Michele died, martin Macneill sat once again before a judge, this time, fighting for his life. This case is a puzzle with many pieces. Reporter: Martin's defense attorneys. What is vpoy imrtant in this case is that none of the medical examiners believe that Michele's death was due to a homicide. Reporter: The jury of five men and three women was about to hear martin's sordid secrets, when witnesses reveal how he really felt about Michele. And he was, "Em I'm glad the

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

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