Jurors will need to determine if the 15-year-old Weier was insane at the time she helped a friend stab a classmate in 2014, in allegiance to the fictional Slenderman character.
It was Slender Man that authorities say motivated the two sixth-grade classmates to stab Payton Leutner 19 times in a Waukesha park. They're not taking issue with Weier having some mental illness, but say she was much more motivated by her fear of losing Geyser as a friend if she didn't go along with the plot.
Forensic psychologist Gregory Van Rybroek, giving evidence at the trial in Wisconsin of one of the defendants, Anissa Weier, said the delusion was so strong about the fictional creature that she "couldn't stop herself".
All three of them were aged 12 at the time.
A psychologist testified during the trial, which started Monday, that Weier was suffering from shared delusional disorder.
Weier and co-defendant Morgan Geyser (GY'-zur) were accused of luring classmate Payton Leutner (LYT'-nur) to a wooded area in a suburban Milwaukee park and then attacking her.
The pair admitted to their roles in the attack but claimed they acted as "proxies" of Slender Man a mythical character peprpetuate don the dark web.
Although Weier did not physically stab Leutner, in her mind she knew it had to be done, Smith told jurors.
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Jury finds Anissa Weier Not Guilty by reason of Mental Illness or Defect. Leutner managed to crawl to the side of a road where a cyclist found her. Van Rybroek is the second court-appointed expert to evaluate Weier's insanity plea.
Weier's attorney, Joseph Smith Jr., asked why Weier, a good student, did not recognize that the belief in Slender Man and his powers to kill them or their families was a delusion.
Waukesha County Deputy District Attorney Ted Szczupakiewicz told the jury during his closing arguments Friday that the girls had planned the attack for months and that they were not in a "kill or be killed situation". She believed he would hurt her and her family if she didn't help commit murder.
The decision means Weier will be sent to a mental hospital rather than prison.
"First of all, she was 12", Westendorf said, adding that the teen was influenced by a website focused on imaginary killers.
He's convinced she legitimately believed that she and her family were in danger from Slender Man.
Both Weier and Geyser were originally charged with being a party to attempted first-degree intentional homicide.
Weier, now 15, pleaded guilty in August to being a party to attempted second-degree intentional homicide.