Approximately nine years after a divorced father went missing in Michigan's Upper Peninsula, investigators are detailing the steps they took to nab the suspect, known as the "devil woman of Michigan," in his brutal murder.

Christopher Regan's 2014 disappearance took Iron River detectives down a rabbit hole that eventually led to the discovery of two murders carried out by a "bored" wife.

In 2017, Kelly Cochran, Regan's lover, was sentenced to life in prison for his murder. A year later, she was sentenced to 65 years in prison for her husband Jason Cochran's murder.

"I was trying to do what I thought was the right thing. You go to school, you go to college, you get married," Kelly Cochran told "20/20" from the Women's Huron Valley Correctional Facility in Ypsilanti, Michigan, in a new episode called "Where Monsters Hide" that aired Friday. 

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She continued: "I got bored from getting married. It was OK at the start, but all I did was work."

Iron River investigators were quickly able to identify Kelly and Jason Cochran, Regan's coworker, as persons of interest in Regan's disappearance, but it took a long time to gather enough evidence to bring charges.

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Former Iron River Police Chief Laura Frizzo told "20/20" that Jason Cochran had motive because he was Kelly's husband and was being "cheated on." The couple had a "troubled" marriage, and Kelly Cochran "was living a life of a single person," Frizzo said.

Investigators conducted a search of the Cochrans' home and found blood splatter on the ceiling, a cracked doorframe and a journal entry from Jason Cochran suggesting a crime had occurred. Investigators later discovered that Kelly Cochran killed Regan, an Air Force veteran, inside the home.

The Cochrans' neighbors told police that not long after Regan's disappearance, they heard the couple using power tools at night. The next day, they invited the neighborhood over for a barbeque with lots of meat, ABC reported. 

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"What if they were actually that kind of people and actually did kill the guy and then butchered him up and had us over for dinner?" neighbor David Saylor said in a police interview, according to ABC.

Private investigators were able to track down the Cochrans in Hobart, Indiana, nearly two years after Regan's disappearance when Kelly Cochran called 911 to report that her husband was not breathing.

An autopsy later revealed that Jason Cochran died of asphyxiation, at which point investigators turned their attention to Kelly Cochran as a suspect in the murders of both men.

She later admitted to injecting her husband with heroin and smothering him.

Hobart Police Department Detective Jeremy Ogden described the different psychological tactics he used to get Kelly to admit to her crimes. The detective told "20/20" that after watching her make frequent visits to a park where she sat by a particular tree, he decided to carve the phrase, "CHRIS IS HERE," into the tree. 

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Later that same evening, Kelly told Ogden for the first time that Regan had been murdered, initially saying that Jason killed him and forced her to dismember his body. She also claimed in court that Jason was abusive.

"I know [Kelly] is capable of murder," Ogden told "20/20." "Everything that I did with her, every interaction, was all noncustodial every single time. And it had to be that way in order to get all the way to the end."

The convicted killer doubled down on those claims in her interview with "20/20," saying Jason tied her up while he killed Regan.

"With [Kelly], it was like ‘Lie, lie, lie, truth. Lie, truth. Lie, lie, lie, truth. You gotta figure it out," Frizzo told "20/20."

Kelly told authorities that she had a pact with her husband that inspired the murders. If either spouse cheated, according to the alleged pact, that spouse would have to kill their lover or be killed by the other spouse, ABC reported.

Kelly pleaded not guilty in the Regan case, but a jury found her guilty on all charges after three hours of deliberation. 

Regan's ex-girlfriend, Terri O'Donnell, previously told Fox News Digital that he was "just wonderful" and "loved life."

"He loved hiking and the outdoors… He was energetic and always wanted to do something new and learn more. He was continually improving himself and going to school," she said. "He even went back to college just so that he could be a better manager at his job."