Laura Greenberg, a journalist driven by an unyielding quest to comprehend the dark motives behind murder, embarked on a chilling journey. Her pursuit led her into the grim world of a forgotten serial killer, incarcerated long ago. As she delved into his confessions, her sense of reality began to blur.

Her unsettling relationship with Doug Gretzler, who, alongside accomplice Willie Steelman, was responsible for the brutal killings of 17 people, including two children, has left a haunting shadow over her life, even decades later. This complex quest for understanding is unveiled in Oxygen’s latest true-crime documentary, “Charmed by the Devil.”

The documentary sheds light on Greenberg’s relentless pursuit to comprehend the psyche of a killer. Over numerous years, she visited Gretzler an astonishing 350 times, exchanged countless letters, and amassed over 500 hours of recorded conversations, all in her pursuit to, in her words, “understand the monster.”

This gripping narrative is at the heart of “Charmed by the Devil,” where Laura Greenberg’s extraordinary connection with death row inmate Douglas “Doug” Gretzler takes center stage. The film reveals the intense dedication and emotional turmoil that defined Greenberg’s quest.

Filmmaker and Nickelodeon voice actor Ben Giroux, who grew up knowing only that his aunt was interviewing a notorious murderer, gained a full understanding of the gravity of her work during the pandemic in 2020. Only then did he listen to the vast trove of his aunt’s haunting recordings, revealing the true depth of her engagement with Gretzler.

“I went and visited Doug 350 times in prison,” Laura Greenberg is heard saying in the documentary. “We wrote hundreds of letters, and we recorded 500 plus hours of audio tapes. I wanted to understand the monster.” (Oxygen)

“She always says there are 17 bodies between them,” Giroux told Fox News Digital. “He was a monster. He was the devil. And yet she was able to look beyond that and establish a human connection. She says she wouldn’t put a label on what that connection was. I’m sure her answer changes daily on how she would define it.”

Giroux noted that the depth of that connection is what makes the story so unnerving.

Laura Greenberg saved hundreds of hours of recordings from Doug Gretzler, including audio heard publicly for the first time in the documentary. (Oxygen)

“When you walk into her house, it’s basically a museum dedicated to this case,” he shared. “There are police reports, crime scene photos, written letters and paintings. It’s a massive trove of communication between her and Gretzler. It’s an obsession. I think they came to both rely on and become codependent on each other in that obsession.”

Gretzler and Steelman landed on death row after a three-week killing spree in Arizona and California. Steelman died of cirrhosis of the liver in 1986 — the same year that Greenberg, a scrappy reporter for City magazine in Tucson, got a tip about Gretzler that sparked her curiosity.

WATCH: FOX NATION DOCUMENTARY HIGHLIGHTS SERIAL KILLER’S MYSTERIOUS AMAZON REVIEWS

Greenberg first contacted Gretzler by letter, hoping to gain his trust. At the time, he refused to discuss his crimes with anyone. Still, her words intrigued him enough to respond. What followed was an unbroken exchange of letters and recorded conversations. Soon after, she visited him in prison.

Laura Greenberg was a Tucson-based reporter who wanted to know why people kill. (Oxygen)

“Gretzler was somebody nobody had ever heard of because the only person he was talking to was my aunt,” said Giroux.

The boundaries blurred quickly. They bonded over their shared New York City upbringings. Greenberg, who was living with a boyfriend at the time, would wait until he fell asleep, then quietly lock herself in the bathroom, draw a bath and, as she later described, listen as Gretzler spoke in detail about the murders. At one point, he professed his love for her and grew increasingly jealous after she married.

Doug Gretzler (pictured here) and Willie Steelman robbed grocery store owner Walter Parkin and killed all nine people inside Parkin’s home outside Lodi, Calif., the Los Angeles Times reported. Among the victims were a 9-year-old boy and an 11-year-old girl. A loved one of the family spoke out in the documentary. (Oxygen)

“No one knew these tapes existed until now,” executive producer Lauren Flowers told Fox News Digital.

Doug Gretzler had been on death row since Nov. 15, 1976, the Los Angeles Times reported. (Oxygen)

“I was surprised by how ordinary some of these conversations were, knowing the intention was to hear about gruesome crimes and uncover the why. Then you put on another tape and hear them talk about the weather or music, what Laura was buying at the grocery store — even realizing they once went to the same music festival and hung out at some of the same places.”

Flowers noted that the documentary raises difficult ethical questions about how close a journalist should get to a story. The unusual access Greenberg gained — and the intimacy that followed — will likely raise eyebrows among viewers, she pointed out. Giroux and Flowers said the film explores Greenberg’s pursuit as professional boundaries are tested.

Doug Gretzler (left) and Laura Greenberg (right) are originally from New York City. (Oxygen)

Gretzler gave Greenberg detailed accounts of his life and crimes. At 22, he fled his responsibilities as a husband and father in New York and headed to Colorado, where he fueled his days with drugs and alcohol. 

Willie Steelman (pictured here), Doug Gretzler’s accomplice, died in 1986 while incarcerated on death row. (Oxygen)

There, he met Steelman, a volatile 28-year-old who had recently been institutionalized. They formed a “pact,” with Gretzler pledging absolute loyalty to Steelman, whom he considered a brother.

Together, they embarked on a cross-country trip with a simple plan — to steal money and drugs. It quickly spiraled into something darker. Gretzler said flatly that it didn’t take much for him to kill.

Laura Greenberg said she doesn’t regret their controversial relationship. (Oxygen)

While some families said the interviews helped answer lingering questions, Greenberg acknowledged that no explanation could ever justify the brutality of the murders.

Douglas E Gretzler, 22, is seen exiting a police car and is led into the police station after his arrest in downtown Sacramento.   (Bettmann Archive/Getty Images)

As Greenberg gathered information, she reached out to loved ones of the victims and to Gretzler’s sister. They opened up to her for the first time.

“On the surface, it’s confusing,” Giroux said. “Wouldn’t they reject someone who became so close to a person who took so many lives and devastated so many families? But I think they’ve come to appreciate her ability to find the truth and dig for every detail. It speaks to her obsessive personality.”

Doug Gretzler reportedly wrote letters of apology to the families of the victims. (Oxygen)

“She got everyone talking — everyone connected with her,” Giroux said. “She has a real gift for getting people to open up. She certainly did that with Gretzler, who rarely spoke to anyone else. She wanted to leave no stone unturned.”

GET REAL-TIME UPDATES DIRECTLY ON THE TRUE CRIME HUB

Douglas E. Gretzler (left), 22, and Willie L. Steelman, 28, are seen here arriving at the San Joaquin County Courts building. According to Oxygen.com, they claimed the lives of 17 people. (Getty Images)

“Does she accept that boundaries were crossed? I think so,” Giroux said. “She’s incredibly transparent and candid about it. She doesn’t shy away from discussing things that would make others flinch. She’s remarkably open about how deep that connection became.”

Greenberg said she felt “disgusted” as Gretzler revealed the darkest details of his murders. He spoke not only about the killings, but also about a chaotic childhood, including the death of his teenage brother. She repeatedly challenged him, making it clear that no explanation could excuse the scale of his crimes.

Laura Greenberg’s nephew told Fox News Digital that she still listens to Doug Gretzler’s recordings. (Oxygen)

Greenberg pushed Gretzler as she studied how violence took root in his life and how he justified his heinous acts. For some of the victims’ families, the interviews clarified how and why the crimes unfolded, filling in gaps that police files and court records never fully answered after years of unresolved questions. It gave them a painful understanding of what happened to their loved ones during their final moments.

“I would fill up the bathtub, and I would listen to him talk to me about murder,” Laura Greenberg is heard saying in the documentary. (Oxygen)

“We wouldn’t have reached anyone without Laura,” Flowers said. “Everyone we found said, ‘Do you know Laura Greenberg? You have to talk to her.’”

“That was especially true with Dee Gretzler, Doug Gretzler’s sister,” Flowers shared. “She was just a kid when she learned about it at school. She lives privately, and this is the first time she’s ever acknowledged that her brother was a serial killer. But she knew that if Laura participated, she could tell her story.”

“Doug and Willie were separate human beings,” retired Tucson Police Detective Weaver Barkman explained in “Charmed by the Devil.” “After they met, this symbiotic relationship formed a third personality. When you put them together, they turn into something that is extremely dangerous.” (Oxygen)

Gretzler invited Greenberg to witness his execution on June 3, 1998. She sat near his sister. In the years leading up to his death, they continued to talk about “everything in life, both profound and mundane,” said Giroux.

Laura Greenberg is in the process of writing a book about her encounters with Doug Gretzler. (Oxygen)

“The finality hit her hard,” Giroux admitted. “What’s so strange is how normal their interactions sounded at the end — almost casual. You’d think the execution would be the end of the story, but I think it was the beginning. Her obsession only grew after that, and her commitment to telling every detail of this story intensified after he was executed.”

What remains unchanged are the lives lost.

United Market in Victo, California, an intended robbery target of Douglas Gretzler and Willie Steelman in 1973. It was closed the night their crime spree escalated into the Parkin family murders. (Oxygen)

“I think Laura got Gretzler to accept that the responsibility was his,” Flowers said. “There are a lot of people with s—– childhoods who don’t kill 17 people — something she reminded him of repeatedly. From the start, we felt a responsibility to the victims and hoped to provide some closure to their families.”

In 1998, Doug Gretzler was executed by lethal injection. Laura Greenberg was present at the prison with his sister. (Oxygen)

“The thing to take away from my aunt’s connection to Gretzler is the danger of obsession — and how we all need to check ourselves,” Giroux said. “You don’t want to go too far down the rabbit hole. This has consumed 40 years of her life.”

“Charmed by the Devil” is available for streaming on Peacock.