For 15 years he was just Uncle John, until she learned he was a serial killer who had murdered her mother

She discovered she was adopted with the help of the man who murdered her mother.

ByAllie Yang and Brooke Stangeland

By the time Heather Robinson was a teenager, she had long known that she had been adopted as an infant, and grew up knowing only her loving family in Illinois.

But in 2000, the then-15-year-old's world was shattered when she learned that the man she knew as her uncle, John Robinson, was actually a serial killer accused of murdering multiple women in the Kansas City area, including her biological mother. Her family was just as shocked as she was.

"When I heard that John had been arrested… I remember [my adoptive mother] running up and down the stairs panicking. 'How could he do this to us? We're going to go to jail. This is horrible. Our lives are over,'" Heather Robinson told "20/20," recalling the day charges for her disappearance and her mother’s murder were added to the complaint against John Robinson.

"That was the first time I ever saw my dad cry," she added.

After her uncle's arrest, the true story of Heather Robinson's adoption began to unravel. She learned that she and her biological mother Lisa Stasi vanished in 1985 when she was just 4 months old, and that their family presumed both were dead.

But in reality, John Robinson gave Heather Robinson, who was born as Tiffany Stasi, to his brother Don Robinson and his wife, who did not know the true circumstances at the time they took the baby in and renamed her Heather Tiffany Robinson.

To this day, Lisa Stasi's remains have never been found.