A nearly four-decades-old cold case is now getting a hot lead, thanks to newly revealed information provided by the Turn To Tara team.

New York prosecutors are now officially looking into allegations from Bill Stevenson, the ex-husband of first lady Jill Biden. He revealed exclusively to News 12 that he had an affair with Robert Durst's wife, Kathie, just days before she vanished.

MORE: The Robert and Kathie Durst Story

The murder mystery involving millionaire real estate heir Robert Durst has been highly scrutinized but kept tightly under wraps by the Westchester District Attorney's Office for weeks. But News 12 can now confirm that the alleged extramarital affair Tara Rosenblum uncovered, along with a witness who's never spoken out before, has become a focal point of this cold case investigation.

Pennsylvania businessman George Kearns tells News 12 he wants justice for the family of Kathie Durst after nearly four decades.

Kearns says he was there the night Stevenson had a romantic encounter with Kathie Durst and even witnessed Robert Durst explode in a fit of rage early the next morning - 10 days before she vanished.

Kearns said he saw "essentially a madman screaming in the apartment."

"He just had no control over himself. He was just out of his mind angry," said Kearns.

The alleged outburst happened in Robert and Kathie's former Upper East Side apartment.

George says Kathie had invited him to crash on her living room couch that night along with Stevenson, after the three of them had gone to a local bar. His story is now in the hands of Westchester's top prosecutor.

One week after News 12's first report aired this month, Kearns says two detectives from the DA's team contacted him and conducted an interview about what he saw that night.

Dan Schorr, a former New York City and Westchester County prosecutor, has tried many homicide and domestic violence cases in his career.

He was working under former DA Jeanine Pirro 20 years ago when Kathie Durst's case was reopened.

Schorr said News 12's interview with Bill Stevenson was probably a tipping point in the investigation.

News 12 reached out to District Attorney Mimi Rocah for comment on the recent news. Her spokeswoman declined, citing the ongoing investigation.

George Kearns says his interview lasted under an hour and now he's just hoping charges will be filed in the case very soon.

Robert Durst is currently on trial for the alleged murder of his best friend in California.

A judge is expected to rule this week on Robert Durst's motion for a mistrial due to his failing health.