First lady Jill Biden's ex-husband, Bill Stevenson, said in an interview this week that he may be "the missing link" in the decades-old mystery case involving Kathie Durst.

Numerous books, as well as a Lifetime movie and an HBO docuseries, have documented the ongoing mystery of Kathleen "Kathie" Durst's 1982 disappearance and speculation that her husband, New York real estate tycoon Robert Durst, 78, is involved.

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Stevenson, a concert-hall owner from Delaware, met Durst while their families were vacationing in the Poconos in the 1960s when they were children, and their friendship continued into adulthood, he told News 12 Westchester in an interview released Thursday.

"I feel like I'm the missing link in this case," he told investigative reporter Tara Rosenblum.

Stevenson said he and his then-wife Jill, whom he was married to between 1970 and 1975, hosted the Dursts at their home for a three-day weekend in 1974. He said the two women connected easily.

"Both students. Both very smart," the concert-hall owner said of Jill and Kathie. "They're similar people at that age. It was hard not to hit it off."

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Years later in the 1980s, after his divorce from Jill, Stevenson maintained his friendship with Kathie and met with her at least five times in New York City to discuss her problematic marriage with Robert.

"I think at that point she really couldn't trust anybody around her who knew Robert, I guess. I don't know why she picked me, but I'm glad she did," he said in the interview, adding, however, that he feels like he "let her down" during the last two months before her disappearance.

He went on to describe a one-night affair with Kathie at her apartment, which he described as "beautiful" until the next morning, when Robert appeared. 

Robert Durst sits in a police vehicle as he leaves a courthouse in New Orleans, Louisiana, March 17, 2015. (REUTERS/Lee Celano)

"The next thing I realize, there is pounding at the door at 7:30 a.m. She runs out of the bedroom, and then runs back in and says, 'It's Bob.' I jump up. I ran out. And then all of a sudden, he screams something. He had a wad of cash rolled up and hit her right in the face with it, and it was so crazy, and he started yelling, 'Kathleen, this isn't going to happen,'" Stevenson recalled. 

He went on to describe the yelling and fighting that happened afterward. Kathie vanished just 10 days later, according to News 12.

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"I got in my car and went to the 19th Precinct, literally within 24 hours. And I said, 'He killed her,'" Stevenson said. "He wrote a couple of things down. He goes, 'We'll get back to you.' I never heard another word from them."

He told the outlet that he "absolutely" believes his affair had something to do with Kathie's disappearance when asked whether he thought so, adding that he did not regret his night with Kathie because "he was abusing her and she wanted out."

"I feel if something was wonderful at the end of her life, I hope this was it. I just hope she had the same thoughts at the end there," he said.

Defendant Robert Durst is shown in an Inglewood courtroom as Judge Mark E. Windham (not shown) gives instructions before opening statements in the trial of the real estate scion charged with murder of longtime friend Susan Berman, in Inglewood, California, May 18, 2021. (Al Seib/Pool via REUTERS) (Reuters)

The New York Police Department could not provide comment on Stevenson's interview to News 12 because it does not keep logs dating back to the 1980s, the outlet reported.

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Authorities declared Kathie dead in 2017. She was 29 the last time she was seen 37 years ago. Robert's homicide trial involving his friend Susan Berman is ongoing in California, and New York recently reopened its investigation into Kathie's disappearance. 

Durst has pleaded not guilty to murdering Berman, who was shot in the back of the head in her LA home in December 2000. Prosecutors say he silenced Berman before she could tell police she helped him cover up the killing of his wife in 1982.

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It took nearly 15 years for police to arrest the New York real estate heir in Berman's killing and another five to bring him to trial, which was further delayed over the last year due to COVID-19.

The Associated Press contributed to this report.