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

Anissa Weier trembled as the jury's verdict was read after a week of testimony and some 11 hours of deliberations. Her attorney said Weier was relieved and cried following the verdict.

"I'm very thankful to the jurors for taking the time to look at what was really going on with her," Maura McMahon said.

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 cyclist 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 Slender Man's "proxies," or servants, and protect their families from the demon's wrath.

Weier, now 15, pleaded guilty to attempted second-degree intentional homicide in a deal with prosecutors in August.

But she claimed 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.

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

Ms McMahon said she hopes the case reveals that children may be dealing with mental health issues lost on adults who have become too busy with their own lives to pay attention.

"Life is better for children when adults around them are in communication with each other," she said.

Judge Michael Bohren ordered a pre-commitment investigation report on Weier and said he would hold a hearing to decide how long to commit her after the report is completed.

He could sentence her more severely than the plea agreement calls for, including up to a 25-year commitment, the same as the maximum prison time she could have received.

In closing arguments, Ms McMahon told the jury that Weier was lonely, depressed and descended into "madness" that warranted a mental hospital rather than prison.

She said Weier's unhappiness stemmed from her parents' divorce, and she latched on to Geyser.

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

Slender Man, a fictional creature of the internet, is a paranormal being who lurks near forests and absorbs, kills or carries off his victims. In some accounts, he targets children. Some renderings show him as a long-limbed, lean man in a black suit, with no face - others with tentacles protruding from his back.

"This sounds crazy, because it is," Ms 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 (Slender Man) except for Morgan's advice and they swirled down into madness together."

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 on October 9.

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