One of two Wisconsin girls who repeatedly stabbed a classmate in the Slender Man case shouldn’t have been tried in adult court, according to an appeal filed by her attorney.

The appeal stated that Morgan Geyser believed a fictional horror character named Slender Man would attack her family if she didn’t kill the girl. This belief made the attack second-degree attempted intentional homicide, a case that belongs in juvenile court, and not the first-degree version of the crime, attorney Matthew Pinix argued in a recent court filing.

Geyser’s appeal also argues that Geyser couldn’t really understand what rights she gave up when she agreed to speak alone with a detective while she was in custody and confessed to the stabbing, The Milwaukee Journal Sentinel reported.

Pinix said it was “a mockery” to suggest Geyser knowingly and voluntarily waived her right to remain silent or to have an attorney. The attorney noted the girl’s age, unfamiliarity with her rights and the legal system, and a mental illness that was later diagnosed.

“It took three months of targeted education (after she was arrested) for Geyser to gain sufficient knowledge such that she understood the legal system and her constitutional rights, including her right to counsel,” Pinix argued in the court motion filed last week.

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Given the circumstances, Geyser’s statements during more than seven hours in custody were involuntary, according to Pinix.

Geyser pleaded guilty to attempted first-degree intentional homicide for the 2014 attack on Payton Leutner, who was stabbed 19 times and left for dead. Investigators said Geyser and co-defendant Anissa Weier lured her from a sleepover to a nearby park in Waukesha. All three girls were 12 at the time.

Leutner survived the attack by crawling out of the woods to a path where a bicyclist found her. Leutner’s mother previously said her daughter has 25 uncomfortable scars on her body.

Geyser, now 16, was ordered to spend 40 years in a mental institution. Weier, now 17, was committed to a mental health facility for 25 years.

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Both Geyser and Weier sought to suppress their statements to law enforcement, but Waukesha County Circuit Judge Michael Bohren denied the defense motions.

The state’s response to the brief is due next month.

The Associated Press contributed to this report.