Defense attorneys for Slendeman stabbing suspect Anissa Weier are working to convince the jury the girl was delusional and believed in the fictional character when she and a friend stabbed a classmate in 2014.
Anissa Weier egged on friend Morgan Geyser as she attacked Payton Leutner so the pair could avoid the wrath of supernatural character Slenderman.
If the jury had not found Weier mentally ill, she could have been sentenced to prison for a decade, according to one of her lawyers, Maura McMahon.
Weier, now 15, pleaded guilty in August to being a party to attempted second-degree intentional homicide but claimed she was mentally ill during the attack and wasn't responsible for her actions.
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".
A psychologist has testified that a 12-year-old girl suffered from a shared delusion with a friend that led to the stabbing of another girl in a Wisconsin park in 2014.
Instead, Weier told Geyser to do it. "Go berserk", was the command, Mr Osborne said.More news: 25 killed in Malaysia school fire
He's convinced she legitimately believed that she and her family were in danger from Slender Man.
Despite being severely injured, Payton, then 12, survived and crawled out of the woods in Wisconsin, US, where she was found by a cyclist with wounds to her chest, stomach and arms.
Because of the plea, jurors on Friday were examining only whether she was legally responsible because of mental illness. Westendorf was appointed by a judge to evaluate Weier, now 15, after her plea.
A second psychologist has testified about the delusional mental state of a girl who's admitted to helping stab a classmate almost to death in a Wisconsin park in 2014.
She added: 'This sounds insane, because it is.
Weier told authorities that she and Geyser believed they had to kill Leutner or else Slender Man would kill them and their families. Jurors saw clips Wednesday of investigators questioning Weier after the attack, during which Weier moved from apparent complete belief in Slender Man, to some doubt, to a realization by the end of her interrogation that she had been duped.
Both Weier and Geyser were charged with being a party to attempted first-degree intentional homicide. But she also pleaded not guilty due to mental illness or defect, setting up the trial on her mental status.