The HBO true-crime series “The Jinx” ended Sunday with a quiet, bone-chilling mutter from real-estate scion Robert Durst: “What the hell did I do? Killed them all, of course.”

And while that may explain the end of Kathleen McCormack, Durst’s long-missing first wife, her story begins in New Hyde Park.

McCormack, a graduate of New Hyde Park Memorial High School, disappeared in 1982 at the age of 29. In 2001, she was declared legally dead.

Durst, the son of a billionaire real-estate magnate, has long been suspected in her disappearance as well as in the execution-style shooting of his close friend Susan Berman. Years later, Durst also admitted to shooting and dismembering a neighbor in Texas, though he was acquitted following a high profile trial in which Durst’s attorneys argued he had shot the neighbor in self defense and chopped him up because Durst suffers from Asperger’s syndrome.

Durst’s was arrested Sunday for the murder of Berman, and the timing coincided with “The Jinx” finale, though series director Andrew Jarecki, said that was coincidental. Following the arrest, James McCormack, Kathleen’s brother who said he believes in his “heart of hearts” that Durst is the killer, told the Daily News, “I’m kind of floating right now.”

“It’s been 33 years of agony, pain and suffering for my family,” McCormack told the News. “Now I’m just hoping and praying he doesn’t come up with yet another magic defense and another dream team of lawyers.”

Kathleen, the youngest of five, grew up in the New Hyde Park home her parents, James and Ann McCormack, purchased in 1962, according to Long Island Weekly.  

In a 2010 Newsday article, her brother recalled driving Kathy to modeling gigs at Roosevelt Field in the 1960s and the day Kathy moved from New Hyde Park to Manhattan with her friend.

James McCormack did not respond to calls and a voicemail left on a telephone number listed as his.

In 1973, Durst married the 19-year-old Kathleen, but the marriage soured and the relationship became abusive, news outlets say.

Kathleen disappeared on January 31, 1982. She was last seen at a friend’s party in Newtown, Conn., at which the friend, Gilberte Najamy, recalled Durst and Kathleen arguing.

Najamy told People Magazine that as Kathleen was walking out the door she said, “If something happens to me, you will check it out.”

“I’m afraid of what Bobby will do,” Kathleen reportedly told Najamy.

Durst waited five days before reporting Kathleen missing, according to reports. When talking to police, Durst said that Kathleen had arrived after the party at the couple’s home in South Salem, where the two argued before Durst drove her to the train back to Manhattan, according to reports. He also said she called again from her apartment later that night.  

But investigators never found any definitive evidence she ever arrived at the apartment, reports say.

Kathleen’s disappearance devastated the McCormacks, her brother James told People.

“We all went through a great period of depression,” he said. “My mom would sit in Kathie’s old room and cry.”

But now, three decades later, James said he believes Durst is ready to confess.

“I believe Bob will somehow have an epiphany of conscience, contrary to advice of lawyers,” he said on the “Today” show Tuesday. “I think he’s about ready to say, you know, ‘Enough is enough. This is what Kathie would want. This is what I’m going to do.’ ”

And a confession has been a long time coming, McCormack said.

“The revelation was like something we’ve been waiting for for 33 years,” he said in the New York Post.