Courtesy of Mark Gerardot (NEW YORK) — Mark Gerardot says he “never” thought his wife of 24 years would be capable of physically harming herself or another person. But on April 23, 2018, she did the unthinkable.On that day, Jennair Gerardot broke into the home of the woman with whom he was having an affair, Meredith Chapman, and shot and killed her before killing herself.“I had to know myself — how she pulled this off, why she did what she did,” Mark Gerardot told “20/20.”The scope of the surveillance and planning he uncovered was far more than he could have ever imagined.Mark and Jennair Gerardot found young love in each other, dating through college and getting married in 1993. In 2017, they were living in South Carolina together.Mark left South Carolina and met Chapman while interviewing for a position in the University of Delaware’s marketing department. He later accepted the job offer and the two soon began having an affair shortly after he began working there, while Jennair Gerardot remained in South Carolina to lease the couple’s home.When she moved to be with her husband in December 2017, she could immediately sense that Mark Gerardot was distant, he said, and she suspected there might be another woman.“She finally asked me, ‘What’s up with you? You’re acting different,’” Mark Gerardot said. “She said specifically, ‘It’s Meredith, isn’t it?’”Although Mark Gerardot would deny the affair at first, his wife continued questioning his relationship with Chapman. Jennair Gerardot would also keep tabs on him and Chapman, he said, and she would know things about them for which he couldn’t understand how.The questioning came to a head around Valentine’s Day 2018. She revealed she had hired a company to gain access to his phone and that it allowed her to read his texts and see his photos and records of all calls he had with Chapman. Mark Gerardot finally admitted to his wife that he had been cheating on her with Chapman.Following these admissions, the two agreed to go to marriage counseling. However, on the day of their second session, Mark Gerardot was putting his jacket on when he felt something in the lining of his pocket.He cut it open and found a recording device had been planted inside by being sewn in. Its flashing light indicated that it was recording him at that very moment.“She told me she wanted to understand the degree to which Meredith and I were in the relationship,” Mark Gerardot said. “She was determined there was a plan between Meredith and I, and she was trying to listen to what that plan was.” In counseling, she agreed to stop planting any recording devices.However, their relationship rapidly began to move toward divorce. Meanwhile, Chapman and her husband were in the process of divorcing her husband, and Chapman had just accepted a new position at Villanova University in the wealthy Main Line region of suburban Philadelphia, and moved into a new home nearby.Jennair Gerardot began to see a divorce coach while Mark agreed to his wife’s requests, including going on hikes and having dinner together, in the coming weeks until he officially moved out.“It was odd, but I’m trying to land this thing so she’s in a good place,” Mark Gerardot said. “If this was what she needed, then I was going to do that.”Mark Gerardot said that although his wife would occasionally break down, for the most part she was “acting normal”. In secret, however, she had been planning revenge.On the evening of April 23, 2018, Mark Gerardot was supposed to meet his wife for dinner so that they could discuss divorce arrangements. But in a series of text messages, Jennair Gerardot told her husband that she would be late and then that she wouldn’t be making it. She then sent more messages.“You ruined my life,” she said in one message, according to Mark Gerardot, who said that the next two read, “I hope you never find happiness” and “Bye, Mark.” Believing that Jennair Gerardot and Chapman had been in a confrontation, Mark Gerardot began rushing to Chapman’s home. He had already been trying to reach her, but his text messages went unanswered.When he arrived, he found Chapman lying face down on her kitchen floor with a pool of blood forming around her head. His wife was also dead nearby, dressed all in black, lying on the floor.Gerardot was shattered. “My world was gone,” he said. “It felt like… suspended reality. It just wasn’t real.”After his wife’s death, Mark Gerardot started poring through her bank statements, phone records and a computer backup in search of answers. The recording device that he discovered in his jacket, he found out, was just the tip of the iceberg.Recording devices“She had planted recording devices in his office,” Dr. Janice Harper, a writing coach working with Geradot on a book, said.Mark Gerardot says he realized his wife had methodically implanted listening devices in many of his clothes and had a daily routine of logging his conversations.“Every day she would take it back out and then sew it back in, download it,” he explained. “She had done that for weeks.”Mark Gerardot says he found that she transcribed “every word of it” in a total of 12 notebooks.Receipts for surveillance equipmentJennair Gerardot also opened a bank account and secret credit cards days after Mark Gerardot admitted to the affair, he said. She used the credit cards to purchase equipment for her elaborate surveillance.In credit card statements, he says he also discovered 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 his and Chapman’s cars.GPS trackers“On the actual GPS maps, there were two icons, one showing my car and one showing Meredith’s car,” he said. “She named my device ‘lying jerk,’ and she called Meredith’s device ‘whore.’”Mark Gerardot’s private chatsThat’s not all. Mark Gerardot says he also found over 400 images of private conversations between him and Chapman on Snapchat.“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 [and] she put my thumb on the reader,” he said.Gun purchase and gun boxIt became clear that while Jennair Gerardot had been seeing a divorce coach, she had also secretly purchased the murder weapon.Mark Gerardot discovered the receipt showing she had bought the gun close to five weeks before she used it. He also found the box for the gun.“The fact that there’s a gun in the closet that I didn’t know existed…it’s unbelievable to think we were headed towards what I hoped out be a civil end,” he said.Her divorce coach Sheila Brennan said she appeared to be thinking about the future, speaking about where she would move to and where she would work.However in hindsight, Brennan says, “It was literally like two different people. It was the person who was planning the future and then it was the person who was planning for the end… She totally had different plans that had nothing to do with divorce. That part is shocking.”Evidence Jennair Gerardot practiced before murder-suicideReceipts reveal she went to a firing range three times before the murder.“The last time she went was just hours before she took Meredith’s life and then her own,” Mark Gerardot said.Cross-referencing records and calendars, he believes he spoke to Jennair while she was at the shooting range.On an audio recording from his car, his wife is heard claiming she was wrapping up and going home “to do taxes” on the day she first visited the firing range.“It’s chilling to hear her act that normal and standing in a gun store,” he said.Chilling letter from Jennair GerardotJennair Gerardot also left behind a revealing letter to her family she had begun writing weeks before the crime, laying out her private thoughts and plans.“They cannot get away with destroying my life for their gain,” she wrote. “My life and soul are irrevocably broken. I am not even a shell of the person I once was.”Mark says he regrets “breaking her heart and making her feel like she had no other choice.”“I could have stopped the relationship… handled it completely differently,” he said.Copyright © 2019, ABC Radio. All rights reserved.