Wife in love triangle murder-suicide kills husband’s girlfriend, then herself: Part 2

Mark Gerardot said he rushed to his girlfriend Meredith Chapman’s home thinking his wife confronted her. He finds them both dead. Gerardot says he’s “still working” on forgiving himself.
8:04 | 09/07/19

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Transcript for Wife in love triangle murder-suicide kills husband’s girlfriend, then herself: Part 2
"Nightline" continues with Amy robach. Reporter: Two months after admitting to an affair that ended his 24 year marriage, mark geradot thinks his wife jennair is coming to terms with their impending divorce. The two planned to meet over dinner to discuss it. When I got there it was so nice outside they had an outdoor patio and I said, "I'm going to be sitting outside." She said, "Okay, great." Jennair texted me and says, "I made a wrong turn. I'm going to be late." 10 minutes or so go by and she texted me back and said "I'm not coming. Just go home." And she sent me a text. It was a photo of trash. There was a condom in the middle of that photo. I knew the context of it. I thought she was outside of Meredith's house going through her trash. Looking for a condom? Looking for evidence of any kind that I had been there. And then she sent me a text that said, "You ruined my life." And then another text that said, "I hope you never find happiness." And a third text that said, meanwhile, I'm already texting Meredith and she's not replying to me. I thought, okay. So jennair went over to her house. There was a confrontation. And so I paid the bill and rushed over to her house. So I walked to the back of the house and I peered in. Reporter: What he saw shocked him to the bone I saw Meredith face down on her kitchen floor. And I rapped on the door and yelled her name. And she wasn't acknowledging me. I went to her and I saw a fleck of blood on her calf. I just couldn't understand why there was blood. So I ran out onto the porch. I hear a commotion outside. And mark is there. He is freaking out. And I said call 911. I open the door I notice that in her left hand she still has her keys which I find odd. So I go around and I feel for a pulse on her body. Reporter: But Meredith wasn't alone. And that's when I saw another figure laying there, and she was dressed all in black from head to toe. And I didn't recognize the face, and yet I did. It was jennair. And I just said to her, "Baby, oh baby. What have you done? What have you done?" I actually heard the radio transmission. It came in as two deceased people in this community, there's not a lot of calls like that. At that moment, my heart just sank. At first I thought we were dealing with a double homicide. As I approached, the -- the front door had a closed in porch. There was an outer door, and then there was an inner door. That inner door had a broken out pane of glass. The one closest to the door handle was missing. So that obviously raises my suspicion. Do we have a homicide where somebody broke into the home and killed somebody? I don't know what I said to the police. He described the one woman as his wife. And the other one as somebody he was having an affair with. I was yelling at them to get in there and help them and there was some woman who was helping me finally just grabbed my hand and said they're gone. And that's where I completely lost it. Finally, the police whisked me off to the police department. Because you were the prime suspect. I was the prime suspect. The murder weapon found was a .357 caliber revolver. Two rounds had been fired from the handgun. Five rounds still in the weapon. The police said we have to ask this question, "Did you shoot these two women?" And I literally said, "Shot them? They were shot?" That's where I pieced it all together. But it still wasn't real. It was determined that jennair had committed a burglary and broken into Meredith's home. Where she lay and wait for her to return from work. And had murdered her before taking her own life. Is there any way to describe what you were feeling? Nightmare. My world was gone. We are learning more about the murder/suicide. By all accounts it appears to be a jealous attack. It became very clear to me when the news broke, that I had seemed to be in many people's eyes public enemy mber one. Mark is in a state of shock and grief. Shunned by his friends he channels all that energy into an obsession to find out what was going on with his wife. Did you ever think that jennair was capable of physically harming? Never. She had never done anything violent. Reporter: But the police uncover evidence that jennair had been meticulously surveilling her husband. I had to know myself, how she pulled this off why she did what she did. Reporter: Mark starts pouring through jennair's bank statements, phone records, computer backup. Turns out the listening devices mark found earlier were just the beginning. 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. Then she actually transcribed every word of it, in notebooks. There were twelve notebooks full of transcripts that she had written. Reporter: Mark finds transactions for a lock-picking kit he believes she used to break into his office. Computer hacking software. 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. Reporter: 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. Reporter: Mark says he also found the receipt for the murder weapon and the box it came in. She bought the gun on March 20. 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. Reporter: When we come back, mark tries to make sense of the unthinkable. I could have stopped the relationship, handled it completely differently. I broke her heart and my regret comes back to making her feel like she had no other choice. There's so much scent in new Gain Scent Blast detergent... ...You'll either love it or... MMM... I guess not. New Gain Scent Blast. Love it or Hate it. It's Intense. When cravings come on strong, be stronger... With New Nicorette Coated Ice Mint. Layered with flavor... It's the first and only coated nicotine lozenge. For an amazing taste... ...That outlasts your craving. New Nicorette Ice Mint. Frustrated that everyday activities cause wrinkles and there's nothing you can do about it? Downy wrinkleguard is a fabric conditioner that helps protect you from wrinkles all day. Pants washed with Downy wrinkleguard and detergent are virtually wrinkle free. Try Downy wrinkleguard.

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

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