A Wisconsin girl who admitted to participating in the stabbing of a classmate to please horror character Slenderman will avoid prison after a jury determined Friday that she was mentally ill at the time of the attack.

Anissa Weier and Morgan Geyser lured classmate Payton Leutner into the woods at a park in Waukesha, a Milwaukee suburb, in 2014. Geyser stabbed Leutner 19 times while Weier urged her on, according to investigators.

A passing bicyclist found Leutner, who barely survived her wounds. All three girls were 12 at the time. Both Weier and Geyser told detectives they felt they had to kill Leutner to become Slenderman's "proxies," or servants, and protect their families from the demon's wrath, said news.com.au.

Weier, now 15, pleaded guilty to attempted second-degree intentional homicide in a deal with prosecutors in August. But she claims she was mentally ill during the attack and not responsible for her actions, in a bid to be sent to a mental institution rather than prison.

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A plea agreement before trial called for her to spend at least three years in a mental hospital if judged mentally ill, and 10 years in prison if not.

The jury's verdict came after some 11 hours of deliberations, and about an hour after it had appeared to reach a verdict in Weier's favour only to see it rejected by Judge Michael Bohren.

An artist's impression of Slenderman. Photo / AP
An artist's impression of Slenderman. Photo / AP

Though that first verdict wasn't read in court, defence lawyer Maura McMahon said 10 of 12 jurors - the minimum required by law - voted Weier was mentally ill. On a second question that jurors had to decide - whether she was criminally responsible for her actions - 10 jurors also voted she was not. But it wasn't the same 10 on both questions, according to McMahon. Bohren ordered the jury to resume deliberations.

In closing arguments, McMahon told the jury that Weier was lonely, depressed and descended into "madness" that warranted a mental hospital rather than prison. McMahon said Weier's unhappiness stemmed from her divorce, and she latched onto Geyser.

Together they became obsessed with Slenderman, developing a condition called shared delusional disorder, McMahon said. Weier believed Slenderman could read her mind as well as teleport and would kill her or her family if she talked about him, she said.

"This sounds crazy, because it is," McMahon said. "This was a real being to this child and she needed to protect those around her. At 12 years old, she had no way to protect herself from (Slenderman) except for Morgan's advice and they swirled down into madness together." Waukesha County Deputy District Attorney Ted Szczupakiewicz countered during his closings that the stabbing was calculated. He said the girls had planned the attack for at least four months. He asked jurors to consider why if the girls were so afraid of Slenderman they waited so long to attack Leutner. He also pointed out that Weier told a detective she wasn't frightened of Slenderman until after the attack, when Geyser told her she had made a deal with the monster that he would spare their families if they killed Leutner. "It comes down to did she have to or did she want to?"

Szczupakiewicz said. "It wasn't kill or be killed. It was a choice and she needs to be held criminally responsible." Weier, bespectacled and dressed in a long grey-and-white cardigan, visibly trembled in her seat during the closings.

Wisconsin law requires only 10 of 12 jurors to render a verdict on whether a criminal defendant wasn't responsible for her actions due to a mental condition. Geyser has pleaded not guilty to one count of attempted first-degree intentional homicide by reason of mental disease or defect. Her trial is set to begin October 9.