His wife and the woman he had been having an affair with were both found dead, their bodies lying on the kitchen floor.
What the police would soon realize was that Gerardot, who was initially their main suspect, had discovered the two women after they were both dead. It was in fact his wife Jennair Gerardot who had shot his girlfriend Meredith Chapman before killing herself.
Watch the full story on "20/20" THIS FRIDAY, Sept. 6 at 10 p.m. on ABC.
Gerardot met Jennair Cox for the first time in the summer of 1986 in Fort Wayne, Indiana. They were both teenagers at the time and she worked at a Taco Bell. Gerardot said he was immediately smitten.
"She made a brown polyester uniform look really good," he told "20/20." "Big, beautiful blue eyes and a big smile. ... I was kind of tongue-tied; I couldn't even speak."
Their romance wouldn't begin until four years later after Gerardot ran into Cox again and found out that she was by then working at a nearby mall. He began visiting her regularly, and one day, he said she pressed him to make a move.
"She said… 'Are you going to ask me out, or what?'" he said. "I was quite shy."
It was this directness that drew Gerardot to Cox. She was confident and unafraid to speak her mind, Gerardot said, adding that her demeanor was "the exact opposite of me."
"When she met Mark, I remember her talking, [saying] this is the one," said Melissa Murphy, a childhood friend of Cox's. "She was very excited. She would do anything for Mark."
The two married a few years later on Oct. 23, 1993. He was 25 and she was 23, and together the two would cement a bond that began in adolescence and carried them into adulthood.
"We were happy," Gerardot said. "I couldn't imagine not being with her for the rest of my life."
But just as with any relationship, Mark and Jennair Gerardot had to deal with low points in their marriage. He said his arguments with his wife could be "epic" and that she would win because "she was always going to have the last word."
"I wasn't privy to all the behind-the-scenes of their relationship and their life together, but I knew there were intense disagreements -- intense reactions to things," said Mike Hartman, one of Mark Gerardot's high school friends.
Money had frequently been a concern and a source of stress for the couple, but when the 2008 financial crisis struck, it "changed everything for us," Mark Gerardot said.
In November 2011, the two moved to Greenville, South Carolina, where they had both found new jobs in marketing. With the new environment, Mark Gerardot said they experienced somewhat of a "renaissance" for their marriage, frequently exploring the state's scenic mountains and beaches.
But by 2017, their renewed life in South Carolina came to a standstill. Jennair Gerardot had been let go from her job and was having difficulty finding a new one. She lost confidence in herself, her husband said. And once again, Mark Gerardot said, the couple decided that it was time to search for new opportunities -- and a new start -- somewhere else.
The interview that leads to Mark Gerardot's affair
Mark Gerardot met Meredith Chapman through an email exchange. In his search for a new job, he saw that the University of Delaware was hiring a creative director in its marketing department. Chapman would be the supervisor. She responded to his email inquiring about the position right away, he said, and interviewed him not long afterward. His concerns that a woman 15 years his junior would be managing him subsided almost immediately.
"When I sat across the desk from her, within five minutes, I didn't say it out loud, but I said, ‘I've got to work for this person,'" he said. "She was so articulate and so energetic and passionate about the job and accomplished at her age to be in the position that she was in. I was dumbfounded."
Gerardot accepted an offer for the position and moved to Wilmington, Delaware, in November 2017. He was excited not only to be moving north but also to see growth in his career, Hartman said. He didn't know that this would also be the beginning of the end of his marriage.
While Jennair Gerardot stayed behind in South Carolina temporarily to lease their home, Mark Gerardot spent his first 45 days living alone in Delaware. During that time, he said he got to know Chapman well. He said the two had a lot in common and one day, she asked him out for a drink.
It was easy to talk to Chapman, Mark Gerardot said of that night, and before they had gone home, he had told her about losing his parents and his brother. He also spoke about his marriage, even if it was "not necessarily in a negative way," he said.
"I really didn't know what was happening," he said. "I knew it felt amazing to be around her, and I was confused by the feelings I was having, to be honest, until it became a little bit more forward with a dinner that we had. It was more of a date."
Mark Gerardot said the issues with his marriage also came into focus when he was with Chapman.
“Here comes Meredith. She's just telling me such a different story, that I'm good at what I do. That she said...I'd never heard this from Jennair's lips ever...that I'm such a wonderful man," Mark Gerardot said. "That just made an impression on me. Here's this woman who I think is amazing saying that she thinks I'm a wonderful man."
Four weeks after he met Chapman, Mark Gerardot said the two kissed. He said that while the energy was there, he "immediately felt awful."
Mark Gerardot told Chapman that "whatever was about to happen" couldn't because he cared for his wife. But he soon realized that he also couldn't give up on the feelings he was developing for Chapman, who he said had told him she was nine years into an unhappy marriage.
"I felt enthralled. It was such a connection. To shut it down just didn't feel right," Mark Gerardot said. "I had to at least find out what it was that...drew us together."
Before Jennair Gerardot had even made it to Delaware, her husband and Chapman had already expressed their love for each other. Mark Gerardot said it "felt right" to tell Chapman that he loved her and that it was a "very powerful feeling."
When Jennair Gerardot moved north in December 2017, Mark Gerardot said she could sense he was distant, and it wasn't long before she confronted him about her suspicions that there could be another woman.
"She finally asked me, ‘What is up with you? You're acting different,'" Mark Gerardot said. "She said specifically, ‘It's Meredith, isn't it?'"
He said he had mentioned Chapman several times by that point, but he said that it had been in the "context of her professional decorum."
Although her husband denied the affair at first, Jennair Gerardot continued to question his relationship with Chapman. Mark Gerardot said she kept tabs on him, and that she mysteriously knew things about him and Chapman. He said he couldn’t understand how she knew.
Near Feb. 14, 2018, Valentine's Day, Jennair Gerardot finally received the confirmation she'd been looking for. But her husband's admission of an affair came at the cost of her own — she admitted to her husband that she had hired a company to gain access to his phone, allowing her to read his texts, and see his photos and records of calls he had exchanged with Chapman.
Mark and Jennair Gerardot agreed to attend marriage counseling. But on the day of their second session, Mark Gerardot said he was putting on his jacket when he felt something bulging in the lining of his coat. Although he initially thought it was an anti-theft device within the lapel, he cut it open and discovered that it was actually a recording device and that, with its light flashing, 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."
From that moment on, the relationship between Mark and Jennair Gerardot would fall apart at an accelerated pace. Mark Gerardot said he told his wife that he planned on filing for divorce in May after he had met Delaware’s residency requirements.
Meanwhile, Chapman, who had been offered a job at Villanova University in Pennsylvania, moved to Radnor Township, Pennsylvania, which is part of the Main Line region. She and her husband were also getting divorced.
As Chapman and Mark Gerardot's relationship continued, Jennair Gerardot began to see divorce coach Sheila Brennan, who said she had a lot of resentment and anger about "being tossed away" and "being traded in" for a younger woman. Brennan said her client also had a lot of "fears about being left financially" since she had just moved north and still hadn't found a job one year after losing her last one.
At one point, Jennair Gerardot even threatened to jump out of the window, Mark Gerardot said. He recommended that she see a psychiatrist and asked his wife to also seek support from her friends and family. He said she took his advice and that she seemed to become more accepting of the upcoming divorce.
When Jennair Gerardot approached her husband with a list of requests for the weeks leading up to him officially moving out, Mark Gerardot said he believed that they had gotten through the worst of things. Among the items on that list: going on hikes, having dinner and generally spending some time together.
"It was odd, but I'm trying to land this thing so she's in a good place," he 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 and cry over the next few weeks, for the most part she was “acting normal."
In secret, however, she had been planning revenge.
Jennair Gerardot's secret plan
On the night of April 23, 2018, Mark and Jennair Gerardot were supposed to meet for dinner to discuss divorce agreements. But in a series of text messages, Jennair Gerardot told her husband, who was already at the restaurant, that she was running late before she sent another one telling him to go home because she wouldn't make it.
Next, she sent him a picture of trash with a condom in the middle, which made him believe she was sifting through Chapman's trash can outside her home. Then, finally, he received more text messages.
"You ruined my life," one text message said, according to Mark Gerardot.
He said the next two read, "I hope you find never happiness" and "Bye, Mark."
When his attempts to reach Chapman by texted went unanswered, Mark Gerardot said he rushed to her home expecting to find a confrontation between his wife and Chapman. But when he arrived, he found Chapman facedown on her kitchen floor with a pool of blood forming around her head. Nearby, he found his wife's body also laying on the floor.
"It took five seconds and I screamed an obscenity and ran to her," he said. "I just said, ‘Baby. Oh, baby. What have you done?'"
Tim Mulvey, one of Chapman's neighbors, was there when Mark Gerardot arrived at the house. He said that he had heard the commotion and that he went out to see what had happened. Mark Gerardot said he told Mulvey to call 911.
Sergeant Christopher Four, of the Radnor Township Police Department, said that the call made his heart sink. "We don't really receive those types of calls," he said. "When we arrived on the scene, it was kind of chaotic. [There were] a lot of officers there."
Mark Gerardot, who Police Chief Christopher Flanagan said seemed to be hyperventilating, was placed into the back of an ambulance before he was brought to the police station as a suspect in the crime. But after questioning him and finding the gun used in the two deaths underneath Jennair Gerardot’s body, the police determined that she had broken into Chapman’s home before murdering her and then killing herself.
In the ensuing weeks, the intricate plan that Jennair Gerardot conceived to spy on her husband and Chapman and eventually kill her came to light.
One day after the murder-suicide, police received a tip from a person who said they had seen a woman in a trench coat, hat, sunglasses and a possible wig scoping out the house with binoculars. Police had also collected a set of car keys from the crime scene, which they determined belonged to a Cadillac that Jennair Gerardot had rented.
Mark Gerardot said learning about the Cadillac came as a surprise.
"I found this out mostly from the police and from the news," he said. "She rented it two weeks before and it was parked right outside our apartment."
The disguise that was described by the neighbor; those items were located in the Cadillac.s
Mark Gerardot said he continued to dig. He pored over his wife's bank statements, phone records and her computer backup and found that "she was living a double life," he said.
After her husband admitted to the affair just after Valentine's Day, he said Jennair Gerardot opened up a secret bank account and credit cards. She used the credit cards to purchase the audio surveillance equipment, he said, as well as a lock-picking kit, computer hacking software and DNA testing kits for his clothes. He said she even bought sophisticated GPS tracking systems and attached them to both Mark Gerardot and Chapman's cars.
"I think there were over 400 images of private conversations that Meredith and I had via text and via Snapchat, and she would actually, in the middle of the night, get access to my phone," Mark Gerardot said. "My best guess as to how she gained access? I was sleeping on the couch. She'd put my thumb onto the thumb reader."
Jennair Gerardot bought the gun that she would use in the murder-suicide on March 20, 2018, five weeks before she committed the crime. Mark Gerardot said that her credit card statements showed she had practiced shooting the gun three times at a firing range.
He said the audio recording device that he found in his jacket wasn't the only one that his wife used, either. He said she had planted multiple devices in his clothes and that he found files containing hundreds of hours of audio that his wife had recorded, even transcribing the audio into notebooks.
"She took all my jackets and had a multitude of devices that she was cycling in and out," Mark Gerardot said. "Every day, she would take it back out and then sew it back in, download it and back and forth. She had done that for weeks."
He said that on the night his wife killed Chapman and herself, she emailed a letter to members of her family explaining her motive "to a very chilling degree."
"She had started the letter weeks before, and she dated her entries. Why [did she do it]? It's as simple as yes, this is payback for what you've done to me," he said. "That's as simple as it gets."
Dr. Robi Ludwig, a psychotherapist who has studied homicide within marriages and read Jennair Gerardot's letter, suggested that Jennair may have been pushed to kill Chapman and herself because losing the love of her life was something she could not tolerate.
"In my opinion, there was something about listening in on the tapes that triggered rage and wound her up," Ludwig said. "It gave her the courage to know that she was right that he was bad."
"I think at some point, when she gave up on her life and became suicidal, that's when she became her most dangerous," Ludwig continued. "I get the sense that all of this was done to make him suffer. She was interested in being the judge and jury in this scenario of her own life and her husband's life."
Now, more than a year after the murder-suicide, Mark Gerardot said that he wishes he handled the breakup with his wife differently.
"I broke her heart and more than anything, [out] of all of this, my regret comes back to breaking her heart and making her feel like she had no other choice," he said. "I wish I wouldn't have hurt her because I loved her. I still love her. I wish I could take it back."
But he is also trying to move forward. He said he has driven past Chapman's home several times and that doing so has helped him come to terms with what happened so that he can let it go.
He said that he also began writing as a means of recovery. In a private letter that he wrote to Chapman, he said he mentions that it should have been him, not her, who died.
As part of his writing, Mark Gerardot has also written a manuscript for a book about the incident. He said that it has been cathartic to do so and that his hope for people reading it will be to learn from his mistakes.
"There [are] a lot of married people out there...going through a lot of the same things Jennair and I were going through. Not to say they're all going to end the way ours did. But...there's a lot of passion, there's a lot of arguing in the book that I think is so common in a marriage," he said.
"We didn't do everything right," he added. "That's for sure."