There are major developments in the Robert Durst murder investigation unfolding in New York.

News 12’s Tara Rosenblum has learned that the ex-husband of first lady Jill Biden, Bill Stevenson, has been approached for questioning by state police investigators. Multiple sources say that a grand jury against Robert Durst will begin no later than next Monday.

Sources tell Rosenblum that the investigators didn't even know of the connection between Stevenson and Robert Durst's first wife, Kathie, until News 12 shared his story several months ago. Now, he could become a star witness in a grand jury proceeding expected to begin any day in Westchester County.

Stevenson, a businessman from Delaware who calls himself the missing link in this case, joined News 12 for a two-hour interview back in the spring. During the interview, he opened up for the first time about the alleged extramarital affair he had with Kathie Durst just days before she went missing in 1982.

Stevenson claims the morning after his lifelong friendship with Kathie Durst took an unexpected intimate turn, Robert Durst barged into the apartment and directed violent threats at her. A week later, she vanished.

Now, investigators reportedly want to hear more of Stevenson's story.

In the meantime, the scandalized real estate heir is behind bars and fighting cancer in California. He is scheduled to be sentenced tomorrow for the killing of his best friend.