Robert Durst, a convicted murder who was awaiting trial on a separate murder charge, died of natural causes while in custody early Monday morning, his lawyer said.
Durst, 78, was sentenced to life in prison in October 2021 for the first-degree murder of his friend, Susan Berman, who was shot and killed in 2000 in her Los Angeles home.
Days after his sentencing, Durst was charged with the murder of his wife, Kathie Durst, who disappeared near their Westchester County, New York, home in 1982. Her body was never found.
Prosecutors allege Durst killed Susan Berman in 2000 because he feared his close friend would reveal details of Kathie Durst's death.
The Westchester County District Attorney's office said it plans to hold a news conference on the Kathie Durst case "in the coming days."
"After 40 years spent seeking justice for her death, I know how upsetting this news must be for Kathleen Durst’s family," Westchester County District Attorney Miriam Rocah said in a statement. "We had hoped to allow them the opportunity to see Mr. Durst finally face charges for Kathleen’s murder."
Robert Abrams, an attorney for Kathie Durst's family, said Monday that he'll provide an update on Jan. 31, 2022 -- the 40th anniversary of Kathie Durst's disappearance.
Durst was also charged in the 2001 killing of his neighbor, Morris Black, in Galveston, Texas. Durst's lawyers argued the real estate heir shot Black in self-defense and then dismembered Black's body. Durst was acquitted.
Durst died of natural causes at 6:44 a.m. local time at an outside hospital, the California Department of Corrections and Rehabilitation said.
Durst's lawyer, Chip Lewis, said in a statement Monday, "We understand that his death was due to natural causes associated with the litany of medical issues we had repeatedly reported to the court over the last couple of years."
Durst was hospitalized in October 2021 with COVID-19. Durst's attorney, Dick DeGuerin, said at the time that the real estate heir was on a ventilator.
In August 2021, during testimony in the Susan Berman trial, Durst detailed his health ailments, including esophageal and bladder cancers, chronic kidney disease and chronic obstructive pulmonary disease, or COPD. He spoke with a raspy, weak voice, impacted by surgery for his esophageal cancer and COPD.
Robert Durst was the eldest son of wealthy New York City real estate investor and developer Seymour Durst. Robert Durst was long estranged from his family and his brother, Douglas Durst, is currently the chairman of the Durst Organization.
ABC News' Aaron Katersky and Meredith Deliso contributed to this report.