Robert A. Durst’s luck may be running out.
Mr. Durst, the estranged scion of a New York real estate family who has long been a suspect in several murders, pleaded guilty in New Orleans federal court on Wednesday to illegally possessing a .38-caliber revolver.
In March 2015, the Federal Bureau of Investigation, fearing that Mr. Durst was about to flee the country, arrested him at the J.W. Marriott Hotel on Canal Street, where he had registered under an alias. During a search, they discovered the handgun. Because Mr. Durst, 72, is a convicted felon, it is illegal for him to possess a firearm. His arrest came as the cable network HBO was broadcasting a six-part documentary about him, which turned him into a notorious national figure.
Under a plea bargain arrangement, Mr. Durst will be sentenced to 85 months in federal prison.
Mr. Durst will eventually be transferred to federal prison in Los Angeles to face charges that he murdered a former confidante, Susan Berman. Under the plea agreement, the prosecution has to arraign Mr. Durst there by Aug. 18.
The resolution of that case could wrap up the mysteries that have enveloped Mr. Durst for the past 34 years.
Mr. Durst is the oldest son of Seymour Durst, whose family owns a dozen skyscrapers in Manhattan. Robert Durst broke with his family in 1994, after his father and uncle picked his younger brother Douglas to take over the business.
Mr. Durst’s first wife, Kathleen, abruptly disappeared in 1982, and the authorities believe that he killed Ms. Berman to ensure she did not reveal what she knew about the case.
Over the years, Mr. Durst, who is worth more than $100 million, has benefited from some of the best defense lawyers money could buy. He was never charged in the disappearance of his first wife.
In 2003, a jury in Galveston, Tex., acquitted him of murder charges, despite his grisly testimony explaining how he cut up the body of a neighbor, Morris Black, and threw the parts into Galveston Bay. The head is still missing.
Mr. Durst claimed that Mr. Black’s death was an accident that occurred while the two men grappled over a gun. Investigators in New York, California and Texas do not believe it was self-defense. Mr. Durst later pleaded guilty to charges of bond jumping and evidence tampering in connection with the case.
Federal prosecutors and investigators from Los Angeles disputed that account and countered that a second, independent search, conducted hours later by Los Angeles detectives, was unquestionably legal.
Mr. Durst’s lawyers were so confident that the federal judge in the case, Helen G. Berrigan, would throw out the evidence that they never formalized a proposed plea agreement that would have meant a sentence of up to 27 months, according to lawyers briefed on the negotiations who were not authorized to discuss them.
Instead, Judge Berrigan sided with the prosecution in October, leaving the defense with little leverage in subsequent plea negotiations.
Mr. Durst’s lawyers say they have been eager to resolve matters in New Orleans so they can get to Los Angeles to answer what they say are spurious murder charges. Once Mr. Durst is arraigned in Los Angeles, the defense has the right to discovery and a look at the prosecution’s case against him.
“They’ve got a TV show and 15-year-old evidence that wasn’t good enough back then,” Mr. De Guerin said, “and certainly isn’t good enough now.”
The “TV show” is “The Jinx: The Life and Deaths of Robert Durst,” which was broadcast on HBO in February and March of 2015. Mr. Durst cooperated in the making of the film, giving the producers more than 20 hours of interviews and turning over reams of court records, phone bills and credit card statements.
At one point, the filmmakers confronted Mr. Durst with strong similarities between the handwriting on a letter he sent to Ms. Berman and on a note the police received after her killing, alerting them to the existence of a “cadaver.”
The documentary concluded with Mr. Durst’s own words, uttered while he seemed unaware that his microphone was still recording: “What the hell did I do? Killed them all, of course.”
Mr. Durst’s defense team will be going up against John Lewin, a Los Angeles County deputy district attorney who has a reputation as a skilled prosecutor of cold cases, including murders. Mr. Lewin has flown to New York repeatedly to interview witnesses, including friends of Ms. Berman and Mr. Durst.
Mr. Lewin also interviewed Mr. Durst for 90 minutes in New Orleans, although the defense will almost certainly challenge the admissibility of that encounter given that his lawyers were not present.
Lawyers are expecting legal battles over handwriting experts and Mr. Durst’s utterances during “The Jinx.” Mr. Lewin must also contend with memories that in some cases are 34 years old.
Ms. Berman and Mr. Durst became fast friends after they met in Los Angeles in the late 1960s. “She was really smart and really interesting,” said Julie Smith, a mystery writer who was close to Ms. Berman. “You never knew what she would say or do. And she had a fascinating background.”
The daughter of a Jewish gangster, Ms. Berman was a promising magazine writer living in New York in 1982 when Kathleen Durst disappeared. Her body was never found. During that investigation, Ms. Berman served as Mr. Durst’s shield against inquiring reporters.
“He was like a brother to her,” said Kim Lankford, Ms. Berman’s friend. “She always spoke of Bobby adoringly.”
But investigators also believe that Ms. Berman knew Mr. Durst’s secrets, which put her in jeopardy in October 2000, when he learned that the authorities had reopened the investigation into Kathleen Durst’s disappearance.
In “The Jinx,” Mr. Durst said that Ms. Berman had called him shortly before her death to say that the authorities wanted to interview her.
Two months later, Ms. Berman was found dead in her Los Angeles home, shot in the back of the head.
Although the police investigation looked at other suspects, it eventually focused on Mr. Durst, who was in California at the time of Ms. Berman’s death.
Ms. Berman, who was in dire financial shape before she died, was fiercely loyal to her friends, Ms. Smith said. Mr. Durst lent her $50,000.
“She described Bobby as the greatest guy in the world and how sweet he was,” Ms. Smith said. “She was not going to rat him out.”