1:19 PM PT -- Durst's attorney, Dick DeGuerin, tells TMZ, "All you have to do is file a lawsuit -- but you have to prove it. And that's where they are going to have their problem."

He also stands firm, "Bob Durst did not have anything to do with Kathy Durst's disappearance and he did not kill Susan Berman and he doesn't know who did."

Robert Durst has just been sued for wrongful death in connection with the murder of his wife.

The Estate of Kathleen Durst is suing Robert, who is currently on trial for the murder of his good friend, Susan Berman, claiming he murdered Kathleen January 31, 1982. According to the lawsuit, obtained by TMZ, the Estate believes based on Durst's alleged M.O. ... after shooting her in the back of the head, he cut her body into little pieces and disposed of the remains ... which have never been found.

The Estate believes the primary motive was his then-lover, "Dear Prudence" Farrow, gave him an ultimatum ... if he didn't divorce Kathleen she would end their affair. The Estate says Kathleen was threatening a divorce and a cut of their fortune. There was also worry she would expose illegal business practices of the Durst companies.

Durst is currently locked up in downtown L.A. where he is awaiting trial in the Berman case.

Durst arguably confessed to both murders on an open mic during filming of the HBO documentary, "The Jinx: The Life and Deaths of Robert Durst." You hear him muttering, "You're caught ... What the hell did I do? Killed them all, of course."