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The family of the still-missing first wife of infamous millionaire and murder suspect Robert Durst has taken a step toward a wrongful death lawsuit against the real-estate heir, it was revealed Thursday.

The brother of Kathleen McCormack Durst filed a petition to become administrator of her estate, according to his lawyer and court documents obtained by NBC News.

Robert Durst; Kathleen Durst Reuters file

Kathleen mysteriously disappeared one night in the winter of 1982 while she was married to Robert. She has never been found and no one was ever charged in connection with the case.

James McCormack is seeking authority over Kathleen's estate in place of their 102-year-old mother, Ann Catherine McCormack.

"The reason James McCormack wishes to be appointed administrator at this time is to commence a possible wrongful death action against the decedent's husband, Robert Durst," Alex Spiro, McCormack's lawyer, wrote in the petition filed in Surrogate's Court of New York County on Thursday.

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"He did it, and we can prove it," Spiro told NBC News.

Durst was widely suspected in Kathleen’s disappearance in January 1982. The two had a tumultuous marriage, and in 1981 Kathleen accused him of physical abuse and filed for divorce.

Durst reported his wife missing and was questioned, but never charged in her disappearance.

Michael Struk, a New York police sergeant at the time, told NBC News in 2008 that while Durst was suspected, "there was never any conclusive evidence as to whether a crime had even been committed."

A body has never been found in the case, but Kathleen was legally declared dead in 2001.

Robert Durst is no stranger to being at the center of mysterious deaths and disappearances. He was arrested this March in Louisiana on an out-of-state warrant in connection with the 2000 murder of confidante Susan Berman in Los Angeles.

"Anybody can file a lawsuit, but you have to have evidence and there is no evidence. There's a craftily edited television show and there’s nothing else."

Durst was arrested the day before the finale of the six-part HBO documentary "The Jinx: The Life and Deaths of Robert Durst." The series delved into his possible connection to Berman's death, as well as Kathleen’s disappearance and the 2001 death and dismemberment of his neighbor Morris Black.

Durst, who became a fugitive and then was caught before the trial, claimed self-defense in Black's death and a jury found him not guilty.

In the finale of the documentary, Durst appeared to confess to the killings as he wore a hot microphone.

While Durst was in the bathroom, he was recorded by HBO saying to himself, "What the hell did I do? Killed them all, of course."

Durst's defense attorney, Dick DeGuerin, told NBC News there was no evidence linking his client to the death of Kathleen.

"Anybody can file a lawsuit, but you have to have evidence and there is no evidence," he said. "There's a craftily edited television show and there’s nothing else."

Durst remains in Louisiana federal prison on gun and drug charges stemming from his March 14 arrest.