Jill Biden's first husband Bill Stevenson says he had an affair with Kathie ...

Jill Biden's first husband Bill Stevenson has claimed for the first time that he spent the night with Robert Durst's murdered wife Kathie ten days before she vanished and that he may be the 'missing link in the case' that drove Robert 'crazy'. 

Stevenson was married to Jill Biden between 1970 and 1975. She met Joe towards the end of their marriage when working for him when he was a Senator, and they got married in 1977 - five years before Kathie Durst vanished in 1982. 

Bill, who previously accused Jill of having an affair with Joe while he was still married to her, was childhood friends with Kathie. 

In an interview with Long Island's News 12 on Thursday, he claims that he and Jill hosted the Dursts at their home in 1974, before their marriage fell apart, and that Robert behaved strangely, talking to himself in the garage. 

He also claims that in 1982, after Kathie confided in him that she was scared of Robert, he spent the night with her at her New York City apartment and their friendship became intimate. 

Stevenson alleges that the next day, Robert showed up at the apartment 'pounding on the door' and that he slammed a wad of rolled up cash in Kathie's face. 

Ten days later, she vanished. 

He thinks Robert - who has long been accused of murdering his wife but has never been charged over it - was driven to kill her after discovering their affair.

'This is something that has to be righted. I was together with them and he went crazy. I feel like I'm the missing link in this case,' he said. 

Stevenson said he watched Kathie 'grow up into a beautiful young woman' after their families became friends when they were kids. 

'She was that cool little sister - I watched her grow up to be a beautiful young woman.'