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.

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.

In an exclusive interview with "20/20," Heather Robinson spoke for the first time about uncovering the disturbing truths about her background, from her real birth date to her adoption by the brother of the man who killed Lisa Stasi. Now, she is on a quest to find the truth about what happened to her biological mother, who was just a teenager herself when she disappeared.

"I want to find out where she is. I want to know who she was," Heather Robinson said. "She was a scared, abused, 19-year-old girl with a newborn, desperate to keep her child [and] be a mother. That was the whole reason John got her... I know I will. I'll find her."

Watch the full story on "20/20" FRIDAY at 9 p.m. ET on ABC

What happened to Lisa Stasi?

Carl Stasi, Heather Robinson's biological father, met Lisa Stasi in 1983.

"She was from Alabama and she was a southern girl, you know? And she was beautiful. Blonde hair, blue eyes," Carl Stasi told ABC News in a 2000 interview.

Lisa Stasi got pregnant and the pair got married, but her aunt Karen Moore told ABC News in a 2000 interview that Stasi had "a fight with Carl" around Christmas in 1984.

"So, I took her [and 4-month-old Tiffany] to the Hope House," a residential facility near Kansas City, Missouri, Moore said.

"At that same time, John Robinson had started… purportedly a program to help downtrodden women," Former District Attorney Paul Morrison, who prosecuted the case against John Robinson in Johnson County, Kansas, told "20/20."

"He had gone to several of these shelters and hospitals telling social workers about this program that he had," Morrison said.

John Robinson connected with Lisa Stasi, and then put her and her daughter up in the Rodeway Inn hotel in Overland Park, Kansas, according to police.

On Jan. 8, 1985, Stasi drove her baby over to the home of her sister-in-law, Kathy Klinginsmith, to help with babysitting and told Klinginsmith about Robinson.

Klinginsmith told "20/20" she "felt uneasy" about their conversation and that the next day, in the middle of a snowstorm, John Robinson showed up at her home and took Lisa Stasi and the baby back to the Rodeway Inn. That was the last time Lisa Stasi was seen alive.

Hours later, Lisa Stasi's mother-in-law Betty Stasi said that she received a strange call from her.

"She just called and she was hysterical. She was crying hard," Betty Stasi said.

Lisa Stasi told her mother-in-law she was made to sign four pieces of blank paper, according to police.

"I said, 'Don't sign nothing.' And she was just crying and then she finally settled down and she says, 'Here they come now'...And she hung up. And that was the last time I heard from her," Betty Stasi said.

"She disappeared forever," Klinginsmith said.

It wasn't long before members of Lisa Stasi's family said they received letters supposedly written and sent by the young mother. One letter said, in part, "I want to thank you for all your help. I've decided to get away from this area and try to make a good life for me and Tiffany."

But Lisa Stasi's late mother Pat Sylvester said in a 2000 interview with ABC News that she suspected something was seriously wrong.

"[Lisa] couldn't type enough to type a letter," Sylvester said. "Did [Robinson] take her baby? ... The more time went by, the more worried we got. The more we began to realize that she might not be alive."

"She would never have let that baby go willingly. Never," Sylvester continued. "He would have had to hurt her or something. She would have fought him tooth and nail."

To this day, authorities do not know exactly what happened on that night or what became of Lisa Stasi, although John Robinson was later convicted of her murder.

"20/20" joined Heather Robinson as she returned to the location where the Rodeway Inn once was -- the last place she and her mother were known to have been together.

"She had everything taken away from her," Heather Robinson said. "Justice and closure for me is finding her remains and giving her a proper burial. Closure is knowing exactly what happened, and peace is finally being able to definitively say Lisa was a person, this is what they did to her, this is where she is and then to be able to finally move on with my life."

Heather Robinson realizes she was once Tiffany Stasi

John Robinson's case as a serial killer broke wide open when he was arrested in 2000 for allegedly sexually assaulting one woman and allegedly stealing sex toys from another woman.

Within days after his arrest, authorities searched Robinson's 16-acre property near La Cygne, Kansas, and a rented storage unit in Raymore, Missouri.

During the search of his property, police uncovered the bodies of Suzette Trouten, 27, and Izabela Lewicka, 21. Then, as they searched his storage unit, they found the bodies of Beverly Bonner, 49, Sheila Faith, 45, and her daughter Debbie Faith, 15. Each woman was found stored individually inside barrels, police said, and Robinson had used Bonner's name to register the storage unit.

In July 2000, prosecutor Paul Morrison announced two new charges against Robinson.

"These amendments include a first degree murder charge involving the death of Lisa Stasi in January 1985," Morrison said. "The second is an aggravated interference with parental custody charge against Robinson involving the carrying away of baby Tiffany Stasi — [she] was purportedly adopted by a Midwestern family in January of 1985 where she remains today."

In a story involving so much violent death, the discovery that Lisa Stasi's daughter had survived and was a thriving teenager offered a glimmer of hope. But the news changed Heather Robinson's life forever.

With the new charges, she also learned that she was being raised by the family of the man who was now accused of killing her biological mother and that she was the missing baby.

"20/20" was there when Heather Robinson recorded an interview with her adoptive father, Donald Robinson, for a potential podcast. During their conversation, Heather Robinson asked him about the night that he learned his brother was a serial killer and that he had been raising one of his brother's victim's daughters as his own.

"It was really the only time I broke down. I had never really broken down before," Donald Robinson said, adding that he was struck by "both" his brother's betrayal and the possibility of losing his daughter to her biological family.

"That was the whole problem — that we had was the possibility of losing you," Donald Robinson told Heather Robinson.

In that conversation, Donald Robinson also told his adoptive daughter that he and his wife had been trying to conceive "for at least five years" before they considered adopting.

Around the time Lisa Stasi went missing, John Robinson contacted his brother, claiming that a woman had killed herself in a hotel room and that her baby was available for adoption.

"Literally that next day, his brother and wife come into town, and there's John Robinson -- without Lisa Stasi of course -- with that baby," Morrison said. "There's a family picture that was taken that day of him with a big grin on his face, bouncing that baby on his knee, with all the other family."

Morrison said that in reality, Lisa Stasi was "probably bludgeoned to death literally the day that kid is handed over to Don."

"Every time I look at that picture I'm thinking like, 'What's he thinking? Was it an hour before he killed your mom? Was it four hours before? Was it the day before?' It couldn't have been very long. And there he is, grinning like a Cheshire cat for that picture," Morrison said.

After John Robinson's arrest, Heather Robinson said she submitted her fingerprints and footprints to the FBI. After matching her prints to those taken at the hospital where she was born, the FBI gave her a DNA test that confirmed she was Lisa Stasi's missing child.

"I don't know how anyone would be able to understand John's mind," said Dave Brown, a former detective who worked on the case. "What satisfaction, what emotional response he might have to see Heather, year after year as she…grows up, and John, knowing full well what had happened, what he had done. I don't know. I certainly can't fathom it."

John Robinson provided "phonied-up paperwork to [his brother], including a certificate of adoption," Morrison said. "Basically he had scammed his brother out of several thousand dollars and these phony adoption fees to get him a baby."

Heather Robinson said the falsified documents were extremely believable; she said her uncle had used the names of a real lawyer, judge and notary. "He was a very good conman," she said.

Heather Robinson acknowledges she "probably witnessed" her mother's murder as a baby, but she said "no one seems to know the details of what actually happened."

"[John Robinson] always gave me this really weird, off-putting feeling in the pit of my stomach," she said. "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."

Shortly before John Robinson was arrested, Heather Robinson said she remembered attending a wedding with him in Florida, where she said he asked her sexual questions and danced too close to her.

"Then he's like, 'I know things are really rough between your mom and dad. I'll tell you what, you message me, don't tell anyone, I'll send you a plane ticket and we will go from there.'"

Heather Robinson said she considered the offer, but added that if she had taken him up on it, "I'd be dead. I would be in that barrel."

She later uncovered even more about her uncle's invented story surrounding her background.

"He had said I was born Oct. 12, 1984. I was actually born Sept. 3," she said. "The October birthday really hurt. I love Halloween and I love having my birthday in October. My first time celebrating the September birthday would have been my sweet 16. I celebrated that locked in my room, crying in bed, wishing I was dead and that I had never been born. That was my sweet 16."

Heather Robinson discovers there are families she never knew she had

After learning that Heather Robinson was alive and well, Carl Stasi and Lisa Stasi's families came forward and asked to meet her.

"I believe if she knows I'm alive she'll want to meet me," Carl Stasi said in a 2000 interview with ABC News. "I feel that… At least [in] the 15 years I didn't know [if she was alive]... I held that in. Now, she's alive and I'm not seeing her."

Meanwhile, the Robinson family that had raised her was concerned about where their custody of Heather Robinson stood as well as how their then-teenage daughter was navigating the barrage of distressing information. Carl Stasi told ABC News at the time that he wasn't concerned with the Robinsons' fears.

"It's my daughter. I want to meet my daughter. My daughter's alive. I have a right to see her, and I want to see her," he said.

Sylvester, Lisa Stasi's mother, saw it differently.

"If our granddaughter is safe and happy and these people have nothing to do with Lisa's disappearance, and they've taken care of this child, we don't want to take her away," Sylvester said. "But we want her to know, that we're her family and we love her."

Heather Robinson developed a relationship with Sylvester.

"My grandma. Lisa's mother…that was hard at times, but I did love her. She taught me to not have hate in my heart and to forgive John…because she did," Heather Robinson said. "I never could understand, and unfortunately, I didn't understand it until after she died, so I never got to have that conversation with her."

She said she decided not to have a relationship with her biological father, Carl Stasi. She was later legally adopted by Don Robinson and his wife, and decided to keep the name they had given her.

"I had been openly asked, 'Well don't you want to know who your biological father is?' I said absolutely not...inside, in my heart, I have a dad, I don't need another one," Heather Robinson said.

Heather Robinson was legally adopted at age 18 by Don Robinson and his wife.

Justice for Lisa Stasi?

In 2002, John Robinson went to trial for the murders of Suzette Trouten, Isabella Lewicka and Lisa Stasi in Kansas.

"The case is just massive. It was over 23,000 pages of police reports. I think we ended up calling well over 100 witnesses," Morrison said.

Heather Robinson's parents did not allow her to attend the trial. Instead, she followed the developments from hundreds of miles away in Illinois.

"It was still just as crazy in Illinois… It got a lot of attention. Because we had people showing up at our house. I know Don lost two jobs from the media attention and them finding out who his brother was," Heather Robinson said. "I think that [is]…why my family and I kind of hid for so long too. I became very defensive and protective."

John Robinson was convicted on all counts in 2002 and sentenced to death.

After the Kansas trial, Robinson went to Missouri, where he faced five additional murder charges. He took a plea deal to avoid the death penalty in that state and pleaded guilty to the five murders.

In addition to Lisa Stasi, two other victims, 19-year-old Paula Godfrey and 27-year-old Catherine Clampitt, have never been found.

John Robinson is now 75 and on death row in Kansas.

Recently, while Heather Robinson was interviewing with "20/20," one of Lisa Stasi's family members gave Heather a 42-page letter purportedly sent from her biological mother in 2010. The letter claimed Stasi was alive and well and that John Robinson would never harm anyone.

Heather Robinson said she is sure her mother is dead and that John Robinson is still trying to string her family along.

Sylvester died in 2018 and the family hopes to recover Lisa Stasi's remains so that they can be laid to rest together. Currently, there is an empty grave with a headstone honoring Lisa Stasi.

Standing at her mother's gravesite, Robinson said she felt "empty, numb [and] defeated" to be there "because I thought by the time I came back here I'd be burying my mother's remains."

"I had hoped that...not only was this journey to help me find answers about Lisa, I wanted to try to heal and give closure to everyone else that was involved," Heather Robinson continued. "I will never stop trying to find her. I will never back down. If I want something I will get it, I will never let go."