It was 1967 when the body of 17-year-old Mary Ann Della Sala was discovered in New Jersey’s Passaic River – and it would not be until 2022 when her family learned the identity of her killer.

Robert Anzilotti, who retired as the Chief of Detectives for the Bergen County Prosecutor’s Office in 2021, elicited a confession from Richard Cottingham, known as "The Torso Killer." The revelation has been kept a secret from the public until now.

A two-part documentary on A&E premiering on Thursday titled "The Torso Killer Confessions," reveals how Anzilotti closed his seventh cold case linked to the tri-state serial killer. The special features never-before-heard audio from Cottingham, as well as in-depth interviews with Anzilotti, as well as loved ones of several victims.

Cottingham, 76, is serving multiple life sentences. 

"Over the years, Richard dropped little hints," Anzilotti told Fox News Digital. "However, we never got too far down the road with it by his own choosing. With Richard, he decides when he’s ready to say something. As hard as I can push him, it depends on the day, it depends on the mood. I had visited him six to eight months after my retirement… I had in my head that this was something I would push him hard to disclose if he was responsible. It was like pulling teeth."

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Mugshot of Torso Killer Richard Cottingham

Richard Cottingham, aka "The Torso Killer" after he dismembered some of his victims, is serving multiple life sentences. (A&E 2023)

"Luckily, he did not know that I was audio recording him," the 53-year-old shared. "We were just having a conversation. I slowly teased it out of him and got him talking about the death without him realizing that he was confessing… He gave great details about the murder and what had occurred. I just couldn’t believe that I got him to finally talk. And I’m not even a cop anymore. I was just happy and relieved for the family because I had long believed he was responsible for this one. It was a cold case that still had not been solved. And I wanted to go back to Mary Ann’s family and tell them, ‘We finally figured it out.’"

Della Sala was a high school honor student and beauty contest winner who was last seen leaving her part-time job as a cashier at a local supermarket. She was last spotted by an employee walking home on a warm night.

Anzilotti has been in touch with Della Sala's siblings, who have yearned for answers over the years.

"All of Richard’s victims were helpless," Anzilotti explained. "They were all beautiful girls and women. They all had their whole lives ahead of them. I think the common denominator for him was he looked for victims that were easy targets. In the case of the many confessions that I got, these were teenage girls. They were either hitchhiking, walking home from school, or walking home from work. In the later years, he graduated to picking on prostitutes and runaways."

A close-up of Robert Anzilotti looking at the camera

Robert Anzilotti retired as Chief of Detectives for the Bergen County Prosecutor's Office in 2021.  (A&E 2023)

Cottingham has been described as one of the country’s most prolific serial killers. The father of three, a former computer programmer for a health insurance company in New York, claimed to be responsible for up to 100 homicides. Cottingham, who has been imprisoned since 1980, was dubbed "The Torso Killer" and "The Times Square Killer" by the press after dismembering some of his victims. His string of slayings dates back to the 1960s.

Cottingham’s DNA was entered into a national database in 2016 when he pleaded guilty to a killing in New Jersey.

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homicide detective hugs Darlene Altman

On Dec. 5, 2022, a homicide detective hugs Darlene Altman, whose mother, Diane Cusick, was killed by serial killer Richard Cottingham in 1968. (Photo by James Carbone/Newsday RM via Getty Images)

"I think the biggest misconception when it comes to Richard Cottingham’s victims is that they were all prostitutes," Anzilotti explained. "In the later years, many of those victims were prostitutes. But the biggest thing that I came across when we connected these crimes to him from his early killing years is these were all just innocent women. These were housewives. These were teenage girls. They weren’t by any means runaways or troubled kids… He murdered across the board. Age didn’t matter. Their lot in life didn’t matter… Many of these victims were just innocent people who were just trying to go through their ordinary lives."

It was 2004 when Anzilotti was tasked with several cold cases dating back to the mid-60s. It was that year when Anzilotti began spending hundreds of hours with Cottingham in what he described as "a cat-and-mouse game."

Robert Anzilotti looking at crime scene clues away from the camera

Robert Anzilotti closed his seventh cold-case linked to serial killer Richard Cottingham, the murder of Mary Ann Della Sala. (A&E 2023)

It took six years before Cottingham confessed to killing Nancy Vogel. The 29-year-old married mother of two was last seen in 1967, when she left home to play bingo with friends at a local church. Her bound body was found days later in her vehicle at Ridgefield Park, New Jersey. She had been strangled.

Anzilotti spent decades talking to Cottingham, coaxing information out of him that only the killer associated with his cases would know.

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Daughter of Torso killer's victim holds up photo of her parents

Randi Childs, daughter of Sheila Heiman, holds up a picture of her mother and father while speaking at a news conference after Richard Cottingham appeared remotely in a courtroom in Mineola, New York on Monday, Dec. 5, 2022. Heiman was found bludgeoned to death in 1973. (AP Photo/Seth Wenig)

"I had an inkling that he was likely responsible for some of these cold cases, but I did not recognize the magnitude and the breadth of his killings until I got to know him," said Anzilotti. "As the months crept into years, my determination got quite steadfast that I was going to keep going until I got him to confess to any of the murders that he was responsible for, at least in my jurisdiction."

"He grew to respect me and trust me," Anzilotti continued. "And that’s what I was going for – to gain his trust. And I think along the way, he developed an admiration for me… Even when I retired, he tried to talk me out of retiring. He kept telling me, ‘Very few people have a love for their job and are as good at it as you are. You’re not going to be happy once you walk out these doors.'"

Robert Anzilotti looking at cold case files

Robert Anzilotti is adamant that there are more unknown victims. (A&E 2023)

According to Anzilotti, Cottingham is connected to 18 murders, and he has no doubt there are many other victims whose cases are still unsolved.

"I don’t know how long he has on this Earth," Anzilotti reflected. "Maybe it’s weeks, maybe it’s years. But I truly hope that we do get that final number and he does confess to everything to whoever he’s willing to confess it to. But my gut tells me we’re never going to know the true number because he’s never going to admit to every single thing he did. I don’t know if he even remembers every single one of these [victims]."

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Family of Richard Cottingham victim

Nassau County District Attorney Anne Donnelly holds a photo of Diane Cusick on June 22, 2022. Richard Cottingham pleaded guilty to the 1968 killing of the Long Island single mother. (AP Photo/Mary Altaffer)

"I don’t think even Richard fully understands what drove him to kill," he continued. "… Richard has always maintained that he had a very normal, happy childhood, that there was no triggering event that created this monster inside of him… He said to me in the past that the biggest thrill for him was the challenge of whether he could kill in all these different ways and avoid law enforcement detection. He maintains that he sexually assaulted and released far more women than he killed. He always maintained, ‘I only killed the ones that I felt were going to get me caught.’"

According to Anzilotti, Cottingham has never shown remorse for his crimes.

A photo of Richard Cottingham from a video link

Convicted serial killer Richard Cottingham, known as the "Torso Killer" and "Times Square Killer," is pictured via video link in Supreme Court in Nassau County, New York, on Monday, Dec. 5, 2022, when he admitted to the 1968 murder of dance teacher Diane Cusick and four others.  (Rebecca White/New York Daily News/Tribune News Service via Getty Images)

"I think the one case that haunts him is the case of [friends] Lorraine Kelly and Mary Ann Pryor because of their ages and the brutality of it," said Anzilotti. "I think it embarrassed him more than he had any remorse for it."

In 2021, Cottingham admitted to murdering Pryor, 17, and Kelly, 16, in 1974.

While Anzilotti ended his 29-year career in law enforcement, the many families he met over the years have never left his mind.

"When you do these investigations, you can’t help but gain an understanding of how traumatic it is for those left behind," he said. "That has always stuck with me. It’s very impactful to be the voice of the victim who’s no longer here. We want to let the families know what happened to their loved ones. And we want them to understand that police don’t give up."

Two-Part Special “The Torso Killer Confessions” premieres March 9 and March 10 at 9 p.m. The Associated Press contributed to this report.