Transcript for Husband uncovers wife’s plan to kill woman he was having an affair with: Part 5
We can't move forward with our lives until I know the whole truth, because you want to protect her until her dying day. You're protecting her to the grave even now. Because you want my information because you want to prepare. I'm sorry what's the question, what's the question? Are we over? Did you ever think that jennair was capable of physically harming? Never. She had never done anything violent. She was an animal advocate. She was an anti-gun person. But the police are finding evidence that jennair has been planning this for some time. A citizen came forward and stated that directly behind the house where the murder took place, that they observed a female in a trench coat, hat, possibly wearing a wig, large glasses and having binoculars. And basically, she was staking out the house. I had to know myself, how she pulled this off why she did what she did. Mark starts poring through jennair's bank statements, phone records, computer backup. And what he finds is Almos impossible to believe. Those listening devices mark found earlier were just the tip of the iceberg. She had told me at some point that all the audio recordings had been destroyed. But then to see them all, hundreds of hours of recordings. She had planted recording devices in his office. She'd took all my jackets and had devices that she was cycling in and out. Every day she would take it back out and then sew it back in, download it. So she had done that for weeks. Then she actually transcribed every word of it in notebooks. There were twelve notebooks full of transcripts that she had written. Just days after mark admits to the affair, she's opening a bank account, secret credit cards to cover her tracks. Mark finds transactions for a lock-picking kit he believes she used to break into his office. Computer hacking software, DNA testing for his clothes and a sophisticated gps tracking system she personally installed on both mark and Meredith's cars. So, on the actual gps maps, there were two icons, one showing my car and one showing Meredith's car. She named my device "Lying jerk", and she called Meredith's device "Whore." She had been watching them for weeks, tracking those two icons wherever they went. There were over 400 images of private conversations that Meredith and I had had via Snapchat, and my best guess as to how she gained access to my phone was in the middle of the night. I was sleeping on the couch. She put my thumb on the reader. Most chilling, mark says, was finding that receipt for the gun and the box it came in. She bought the gun on March 20th, close to five weeks before she used it. It's unbelievable to think we were headed towards what I hoped to be a civil end. So while jennair has hired and is seeing this divorce coach she has already secret ly purchased the murder weapon. She talked about her options of where she would move to and employment opportunities. So she sounded like someone who was planning for a future, absolutely. But it was literally like two different people. So it was the person who was planning the future and then it was the person who was planning for the end. There were three times where she actually went to a firing range. The last time she went was just hours before she took Meredith's life and then her own. As mark is cross-referencing records and calendars, he believes he was speaking to jennair while she was at the shooting range. What are you doing tonight? I just got done finishing those errands and I'm heading back home to do taxes, so. I'm hungry I'm going to set to eat, and finish it up. It's chilling to hear her act that normal and standing in a gun store. Do you have any answers? I think she just wasn't well. And I -- as I look back seeing things from the very beginning that should have been signs. She wrapped her entire life around mine. She wanted us to be on an island together and she wanted to control that. She left behind a letter that she appears to have begun writing weeks before the crime, that lays out her thoughts and her plan. Jennair starts out by writing, "I really enjoyed being a wife and partner. Being a caretaker for my family and household was very fulfilling to me." I got the sense that she really had one route to happiness. And it was through her husband. We brought the letter to Dr. Robi Ludwig a psychotherapist who has studied homicide within marriages. I think it's very hard to see somebody's failed psyche sometimes on the outside especially when they're so invested in presenting a pretty picture to the world. In my opinion, there was something about listening in on the tapes that triggered her rage and wound her up. Just know I'm thinking about you and I'm sorry. Thanks. And it gave her the courage to know that she was right, that he was bad. In the letter jennair goes on to say, "Mark and Meredith cannot get away with this." I think at some point when she gave up on her life and became suicidal, that's when she became her most dangerous. "I want justice, I deserve respect, I want control, I need to take matters into my own I get the sense she was interested in being the judge and jury in this scenario of her own life and her husband's life. It's as simple as this is payback for what you've done to that's as simple as it gets. So instead of killing you, killing your girlfriend would be a way to harm you the most. Correct. Do you think you could've stopped it?
This transcript has been automatically generated and may not be 100% accurate.