Terri O’Donnell instantly knew her ex-boyfriend Christopher Regan had gone missing. However, she was not prepared to discover he had been brutally murdered by his married mistress.

The 55-year-old participated in a new documentary on Investigation Discovery titled “Dead North,” which she hopes will shed light on the man she knew — not the tragic victim he became.

The docuseries chronicles the investigation through shocking interviews, extensive bodycam footage and interrogation room conversations.

Kelly Cochran, the alleged mistress, was sentenced to life in prison without parole in May 2017 after an Iron County, Mich., jury convicted her in the killing of Regan three years before.

Kelly Cochran who admitted to injecting her husband with a lethal dose of heroin may have killed nine other people and served her lover’s remains at a barbecue. (Lake County Sheriff’s Office)

Then in April 2018, she was sentenced to 65 years in prison for slaying her husband Jason Cochran.

Cochran, then 35, told police she delivered an overdose of heroin to her husband, 37, and proceeded to put her hands on his neck, nose and mouth until he died less than a minute later.

As for the Regan murder, Michigan prosecutors maintained she lured the 53-year-old to her home, where her husband shot him. Then they both dismembered him and hid his remains in the woods.

O’Donnell told Fox News her last conversation with Regan still haunts her.

“When I talked to him the last time, he told me that weekend we were going to get together for supper,” she explained. “I tried getting a hold of him and he never responded. And that was very unlike Chris. He would message you right away.

Terri O'Donnell (right) with Chris Regan. (Courtesy of Investigation Discovery)

"And when he made plans, he didn’t break them. It was so out of his personality not to respond… There were rumors among our friends he had went to see this woman name Kelly who was married. And that maybe he was with her. But that didn’t matter. He would have responded right away.”

O’Donnell first met Regan in the ‘80s when he was in the Air Force. The two called it quits and then reconnected in December 2012.

The former couple wouldn’t start dating again until February 2013, but split once more in April 2014. O’Donnell said she and Regan remained close until he was reported missing in October.

“Christopher was just wonderful,” she recalled. “He 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… He even went back to college just so that he could be a better manager at his job.

“He also enjoyed being at home and preparing a fancy dinner. He loved to sit and just converse with you. He had these beautiful blue eyes that would just make you melt. He looked at you like you were the only person that mattered. And he made you feel that. He made you feel like you were on a pedestal all the time. It was an amazing feeling.”

Chris Regan (left) with Terri O'Donnell. (Courtesy of Investigation Discovery)

O’Donnell said that they never spoke about dating other people. O’Donnell claimed she didn't hear about Cochran until Regan went missing. She also insisted that, by the end of August, Regan wanted to get back together again.

“We were spending time together and I would tell him that we needed to take things slow and just be friends and see if things worked out or not,” she explained. “He would send me texts saying… he loved me. I just kept telling him… we gotta take our time. In September he sent me the song ‘Marry Me’ by Train. He was such a romantic.”

On October 14, Regan was getting ready to pack up and start a new job in Asheville, North Carolina. O’Donnell described him as excited to start a new chapter in his life. The pair had made plans to visit each other during Thanksgiving or Christmas.

Instead, a nightmare unfolded.

Michigan prosecutors alleged that the Cochrans, who married in 2002, made a pact on their wedding night that if one of them was unfaithful, they’d kill their lover.

Kelly Cochran (Courtesy of Investigation Discovery)

The couple moved to Michigan in January 2015. Cochran testified in court their marriage was allegedly falling apart and she had affairs with other men, including Regan.

According to court records, she lured Regan to her home with the promise of sex when her husband interrupted them and shot Regan.

Investigation Discovery said in a statement that Cochran’s own brother, Colton Caboyan, told investigators he feared his sister was a serial killer and neighbors said they believed they were served human remains, possibly Regan’s, at a barbecue.

One friend who participated in the documentary described eating a strange-tasting burger without realizing at the time it could have been human.

Kelly Cochran (Courtesy of Investigation Discovery)

O’Donnell admitted the horrific details of Regan’s death kept her up at night.

“I would go kayaking at night,” she said. “I really just wanted to know what happened to him. But the more and more that came out, like mutilating his body… I had to push it out of my head… I stopped reading the newspaper. I couldn’t read comments online… I guess it just made it more real.”

The Iron County Reporter revealed police targeted Cochran because she was one of the last people to see Regan alive. However, when police searched her home in March 2015, they found nothing. Still, she and her husband moved to Indiana.

But when Jason died in February 2016, evidence revealed it wasn’t just caused by an apparent heroin overdose. Michigan authorities charged Cochran with Regan’s death and then she fled again.

The U.S. Marshals Service eventually tracked her down in Kentucky, where she was arrested and taken into custody. Court documents revealed Cochran spent her time in jail turning her glasses into shanks and threatening to commit suicide, as well as bodily harm to anyone who came near her.

O’Donnell said she was terrified of Cochran.

Kelly Cochran and Jason Cochran on their wedding day. (Courtesy of Investigation Discovery)

“The first time I saw her was in the courthouse,” said O’Donnell. “I just remember her staring at me and grinning. I took a deep breath and thought she was the scariest person I've ever seen. I was afraid. I couldn’t look at her for the rest of the time I was there testifying. It was like she was… laughing and saying, ‘Look at what I did. You can’t stop me.’… She just sat there and grinned.

“It was like the devil looking at you… And after I testified… I lost it. I totally lost it. I just remember driving, speeding, not knowing if I had crossed the border. I didn’t know if I was in Wisconsin or Michigan… I didn’t know what to do. I just wanted to run as far away from Kelly as possible.”

Cochran directed Michigan authorities to a site where parts of the rifle used to shoot Regan were found, as well as a .22-caliber bullet. Human remains were also discovered, including a skull with an apparent bullet hole. A pair of eyeglasses, believed to belong to Regan, were nearby.

O’Donnell insisted Regan’s murder was not the first for the Cochrans.

“I do not think that Chris is the first person that they murdered,” she explained. “There’s just no way that the first person you choose to kill, you’re going to be able to clean up the blood and get rid of the body so that FBI agents can’t pull DNA off the walls… To do what they did to him? I don’t know how you could do that if it was the first person you’ve ever murdered.

Investigator Laura Frizzo searches the woods in Michigan's Upper Peninsula. (Courtesy of Investigation Discovery)

“Shooting someone, that’s one thing… You can be far away, pull out your gun and it’s done. But to sit there, take a cut up a body and to then wrap his parts, put it in your vehicle and haul it over to the woods and bury it? There’s no possible way you could logically think through all of that… I truly believe there are other people out there.”

The network added per the provisions in Cochran’s plea, the State of Indiana can never charge her for additional murders. If she chooses, Cochran can provide locations of other victims without any penalty.

She claimed to having other “friends” buried in Indiana, Michigan, Tennessee and Minnesota. However, the identities and specific locations of these bodies remain a mystery to this day.

O’Donnell said she is slowly moving on with her life and is even dating again, but still has a hard time trusting others.

“I question people’s motives a lot,” she admitted. “I’m very aware of people around me… Every summer since Chris’ murder, I’ve traveled out of the country. I don’t know if that’s a coping method.

Investigator Laura Frizzo searches the woods in Michigan's Upper Peninsula. (Courtesy of Investigation Discovery)

"I remember when they were looking for Chris, during the daytime I would walk in the woods, just trying to find something. A piece of his clothing. Just anything. I don’t want to do that anymore. He’s still not completely found. He’s around the county. Which is scary.”

"Dead North" is available for streaming on IDGO.com.