Melanie McGuire is convicted of husband's murder after lover testifies: Part 2

Dr. Bradley Miller, the man McGuire had been having an affair with while she was married, was the prosecution's star witness. McGuire is serving a life sentence and has exhausted all appeals.
7:57 | 09/26/20

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Transcript for Melanie McGuire is convicted of husband's murder after lover testifies: Part 2
Six months after bill Maguire's bullet-riddled, dismembered body washed up in suitcases on Virginia Beach, investigators turned their attention to New Jersey where he and his wife had last lived and he was last seen alive. The task force had local police, state police, the attorney general's office. They decided to treat this case with top priority. Police searched for a record of the gun used in bill's murder, turns up, a purchase made by Melanie in Pennsylvania. Because bill Maguire had a felony conviction stemming from his horrendous driving record, he was barred from purchasing firearms. You say your husband asked you to purchase the gun. But you purchased the gun just two days before your husband went missing. Of course it's a coincidence. He was on me for a while about that. If he was in some sort of trouble, that may very well been the reason why he was so intent on getting it. As police hone in on Melanie, they begin wiretapping her phones, discovering her affair, providing a possible motive for the murder. Eager to prove his innocence, Dr. Miller agrees to wear a wire. You swear you had nothing to do with this? Yes. On your children's lives? Because I'm standing by you. Yes. Investigators also focus on Melanie's highway toll records, trying to track her movements in the days surrounding bill's disappearance. On April 30, 2004, the day after the couple's alleged fight and the day Melanie filed for a restraining order, she says she goes to Atlantic City looking for bill's car. Absolutely not rational, but get in the car and go down there. And lo and behold, there it is. She decides she's going to move his car. Melanie says in the past, when they're angry at each other, it's a way of messing with each other I guess you could say. We believe she was planting his car in Atlantic City. She was trying to create an illusion that bill Maguire was still alive. Investigators also find that five days after bill went missing, Melanie took a trip to Delaware. She says she was furniture shopping. We believe that that was when she took the suitcases with bill's remains and got rid of them in the Chesapeake bay. And the first suitcase turned up the very next day. The circumstantial evidence against Melanie is piling up. But investigators need physical evidence to link her to bill's murder. So they scour the couple's townhouse. They couldn't find any forensic evidence whatsoever to establish this was a crime scene. I don't know how you would dismember a body and effectively clean that up. Still, investigators decide to arrest Melanie on the circumstantial evidence, more than a year after bill's murder. Her 2007 trial becomes a media circus. The evidence in this case points to a well-organized, meticulously-planned, execution of a murder. Were you somebody if you were going to do something you were going to do it all the way? Correct. If that's the case then I would have been sure not to include my own luggage. I don't get to be an evil genius and an idiot at the same time. Prosecutors present evidence they say was key to Melanie's claim, including a forensic examination of the home computer searches. How to purchase guns illegally. How to commit murder, and undetectable poisons was searched. I was a nurse. If I wanted to look for something like that, I have a book that I can look in that doesn't leave an internet history. And then the evidence that was referred to as human sawdust. Microscopic pieces of bill Maguire's flesh investigators had found in his car. That theory was that this nurse, who was kind of so maparticularless accidently tracked these human sawdust particles of bill's body from the house to the car. The prosecution's star witness, Dr. Bradley Miller and the secret recordings he'd made for investigators. After you tape recorded her, sir, you then had additional intimate relations with her, correct? Yes, sir. I think that was damaging to his character. It was a very big turnoff to the jury. We looked very closely at Dr. Miller. There was no evidence connecting him with this crime. How do you find as to the count of the indictment charging Melanie Maguire with the murder of William Maguire? Guilty. Melanie Maguire is now serving a life sentence at a correctional facility in new Jersey. Having exhausted her appeals, she reached out to criminology professor Megan sacks. She had done poorly in the justice system and was frustrated. After visiting with her the first time, I was like, this is a story, this is important. In 2019, sacks and her friend decided to make a podcast. This was the first time someone was basically saying "We hear you." The series looked into the gun that was used to kill bill, which was never found, and the trash bags the prosecutor said matched the trash bags Melanie used to throw out bill's clothes. He ran tests, but he didn't do all the tests that you actually need to definitively say whether these bags matched. They put on a really good show. When you have props, two garbage bags, oh, wow, those do look alike. Based on the testing we conducted and results I observed I felt confident at the time to believe that those bags were created at the same run. And questions about animal hair inside the suitcases. They looked high and low to connect Melanie to some pet, and once they found out that there was no way to connect Melanie to these pet hairs, it became not of evidentiary value. You mentioned that you found some animal hairs. Did you find anything that you considered of evidential value? No. Melanie did not incapacitate, shoot, use a saw to dismember her husband, and also, if the crime scene didn't happen there, and she's home with her children all night, where is this happening? There are just too many holes in this story. This was a murder trial. It wasn't a contest to see who could outsmart the investigators and if someone is smart enough to successfully hide a crime scene that they win. Did you find any evident that exonerates Melanie? I wished we did, I had hoped we would. I still think we might get a tip-in that direction. What do you say to the people at home who watch this and say how dare she talk from prison and bring this all up again? The killer is out there, and it's not me. There are times when and media can shine a light where what the government did was not right. This is not one of those cases. I think bill deserves to be remembered as fantastic friend. A dad who loved his boys. He didn't deserve this.

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

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