Viewers of a upcoming new true crime series have been warned they 'may never know the truth' about an adopted child who turned out to be a fully-grown adult.

We are, of course, talking about the infamous docu-series The Curious Case of Natalia Grace, which explores the mind-blowing story of Natalia Grace, a 22-year-old who posed as an adopted six-year-old Ukrainian orphan.

Natalia has a form of dwarfism called spondyloepiphyseal, with her case being compared to the movie Orphan, which sees a fully grown adult with shunted growth pretend to be a child in order to infiltrate a family.

Natalia went on to be adopted by Michael and his wife, Kristine in 2010 in the US.

They went on to claim, however, that she 'threatened to stab' their children while living with them in Layfette, Indiana, and also tried to 'poison and kill' Kristine.

Michael said: "One night, I opened my eyes and Natalia is standing at the foot of the bed with a knife in her hand."

Investigation Discovery

Kristine also claimed Natalia was already getting periods and had adult teeth, while medical tests looking at Natalia's bone density indicated she was much older than the family initially thought.

After these shocking incidents, the family then began to grow doubtful over Natalia's identity and suspected that she was actually an adult illegally posing as a child.

Despite this evidence, the couple were eventually charged with neglect of a dependent after taking off to Canada and leaving Natalia to fend for herself - but not neglect of a child, due to the court-ordered age change.

Natalia has since moved on with a new adoptive family.

However, Michael was found not guilty of three counts of neglect and conspiracy to commit neglect of a dependent, while the charges against Kristina were dropped in March.

Now, ahead of the docuseries being released next week, attorney Beth Karas joined TMZ Live to speak about the first episode of The Curious Case of Natalia Grace.

Investigation Discovery

On being asked if the family went back to Ukraine and tried to find birth records or parents, Karas said: "They went to the court with an emergency petition and an affidavit from a doctor who had been treating her for the past few years who said that he did not notice any growth and there was no growth in four years.

"She was supposedly eight at the time. So he said, 'Well, you stop growing when you're 18. If she hasn't had any bone growth in four years, then she's at least 22'.

"And with a stroke of a pen, he went from age eight to 22 for Natalia. So she theoretically became somebody who could vote, drink, and drive."

Karas further gave her opinion on being asked how a six-year-old child posed as a 22-year-old woman can actually pull off something so deadly.

"There are people who observed her who lived near her because she had two different apartments, and they come down on a different side whether or not she is an adult or a child.

TMZ

"They observe her riding a bicycle with another child, they also observe her acting out and coming out on men sexually," she added. "So, was she masquerading as a child?

"I'm not so sure because we are the sum total of our life experiences and we don't know what her life experiences were until then, even as a child, maybe she was sexually abused as a child, and was acting out because she didn't know any better."

Karas added: "She didn't know her boundaries. But it's very strange because neighbours who had a lot of contact with her some believe she was an adult and some believe she was a child, and really the viewers are gonna make up their own minds because I really don't know."

"I lean one way but I don't know we'll ever know the entire truth here."

The first part of The Curious Case of Natalia Grace will be available to watch on Investigation Discovery later this month on 29 May.