Woman who learned her uncle killed her mother breaks silence

Heather Tiffany Robinson shares her story in the “20/20” exclusive, “Sole Survivor,” almost 20 years after the shocking revelation.
2:59 | 10/04/19

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Transcript for Woman who learned her uncle killed her mother breaks silence
We'll shift gears now to our "Gma" cover story this morning. A "20/20" exclusive. Shocking story about a bomb who finds out her true identity years after her adoption and what really happened to her biological mother. Amy is back. You sat down with this woman. This is an incredible story in yeah, this is the first time that she's been willing to talk about this incredibly traumatic event. Now, nearly 20 years after Heather Tiffany Robinson learned her actual real identity, she is finally ready to break her silence and share her story. It was a disappearance that baffled investigators. A mother and child who vanished without a trace 15 years ago. Reporter: In 1985, 19-year-old Lisa Stacy and her 4-month-old daughter Tiffany The body of Lisa Stacy has never been found. Reporter: After 15 years a break in the case, baby Tiffany is found. Tiffany Stacy was purportedly adopted by a midwestern family in January of 1985 where she remains today. Reporter: The young girl now known as Heather adopted into a loving family, but how she ended up there, shocking. Heather's adoption was arranged by uncle John, serial killer John Robinson convicted of killing eight women including her mother Lisa Stacy. Now, for the first time ever, Heather is telling her story. No one knows exactly what happened obviously in that motel room but it's highly likely that you were right there in that room when John murdered your mother. If not murdered at least taken from her. No one seems to know the details of what actually happened. I probably witnessed her murder. I still remember that picture. Yeah if on the day he brought you there with your folks in it and you're bouncing on his knee. Uh-huh. You know, every time I look at that picture, I'm thinking what is he thinking? Was it an hour before he killed your mom, four hours before, the day before and there he is grinning like a Cheshire cat for that picture. Yep. Reporter: Heather's adoptive parents were unaware John killed Heather's mother. What were your first memories of uncle John? I don't remember the younger years. I just remember the feelings. He always gave me this really weird offputting feeling in the pit of my stomach. It's like walking down a dark alley in the middle of the night while you know someone is behind you approaching you closer and closer. Reporter: Now devoted to finding her mother's remains and laying her to rest. I will never stop trying to find her. Reporter: John Robinson was convicted and sentenced to the death penalty. He is still behind bars where he will live out the rest of his life there but Heather wants so desperately to talk to him and wants to know what happened to her mother and where she is. So chilling. Okay, Amy. See "Sole survivor" twonts at 9:00 P.M. Eastern right here on ABC. That will be something.

This transcript has been automatically generated and may not be 100% accurate.

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