An American couple who adopted their daughter in 2010 believing she was 6 years old says they believe she's actually an adult woman posing as a child — and that she's hell-bent on killing them.

Kristine Barnett, 45, and her now-ex-husband Michael Barnett, 43, were recently charged with felony neglect of a dependent after abandoning their adopted daughter, Natalia, who suffers from a rare form of dwarfism, in their Indiana home and fleeing to Canada in 2013 to start a new life with their three biological children.

The pair says their concerns about Natalia began shortly after they adopted her. They told the Daily Mail they noticed the girl seemed to have an advanced vocabulary for a child that was supposedly 6, and that she had already exhibited signs of puberty, including a menstrual cycle and "full pubic hair."

Photos of the family:

Things took a turn for the worse when Natalia began exhibiting violent behavior, primarily towards her adoptive mother, the Barnetts told Inside Edition.

Natalia allegedly told her new parents she wanted to stab the entire family to death in their sleep and tried to kill Kristine Barnett twice, once by shoving her into an electric fence and again by pouring cleaning fluid into her coffee.

"She had poured the Pine-Sol in her coffee and just said blatantly, plainly, no emotion, "I'm trying to kill you, I want you dead," Michael Barnett claims.

The couple eventually sought a medical professional's opinion on Natalia's age and were told in 2012 by their primary care physician, Dr. Andrew McLaren that the girl's 2003 birth date was clearly inaccurate, according to the Daily Mail.

Two years after adopting the child, the couple applied to have Natalia's age legally changed so she could receive "appropriate psychiatric treatment for an adult" and were granted the appeal by Marion County Superior Court Judge Gerald S. Zore. The supposed 8-year-old girl's birthday was then changed to September 4, 1989, making her legally 22 years old.

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The Barnetts left Natalia behind in Indiana shortly after, prepaying a year's worth of rent on her apartment and signing her up for social welfare programs.

However, according to an arrest affidavit, a bone density test carried out on Natalia in June 2010 concluded she was approximately 8 years old, just 2 years older than her supposed age when she was adopted. A skeletal exam carried out in 2012 at the same hospital concluded she was about 11, the affidavit says. Her adoptive parents vehemently deny the claims.

Both Kristine and Michael Barnett surrendered to arrest warrants served in late September 2019 and have since been released on bail.

The case has drawn widespread comparisons to the 2009 film "Orphan," in which a couple adopts a 9-year-old Russian girl and later discovers she is, in fact, a 33-year-old woman who has killed at least seven people, including the last family who adopted her.