As part of the agreement, the jury was instructed to decide whether Weier's mental condition would have made her legally responsible for her actions during the attack.
A total of 10 out of 12 jurors agreed on Friday that Weier had a mental disorder when she and her classmate Morgan Geyser stabbed their sixth-grade classmate 19 times.
A bicyclist was passing through the park and found Leutner into the woods, she was badly injured and there was blood all over.
Mr Van Rybroek's expert testimony matched that of another psychologist who said Weier and Geyser had a shared delusion about Slender Man. All three girls were 12 years old at the time of the stabbing, three years ago.
"Anissa was under the command and control of a delusional disorder". 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.
The psychologists viewed video of Weier's confession after the crime.
Slender Man, a fictional creature of the internet, is a paranormal being who lurks near forests and absorbs, kills or carries off his victims. Although Weier did not physically stab Leutner, in her mind she knew it had to be done, Smith told jurors. Van Rybroek is the second court-appointed expert to evaluate Weier's insanity plea.
"She would have an uphill road", McMahon said. Weier and Geyer felt like they had to kill classmate Leutner.
A young schoolgirl has helped her friend stab her own classmate for as many as 19 times, a development which has caused quite stir.
Morgan Geyser's trial is scheduled to begin later this month.
The Journal Sentinel reports Fisher testified Wednesday that Leutner told her Geyser had become obsessed with Slender Man and other scary characters on the Creepypasta website.
Weier will now be admitted to a mental institution.
Geyser's trial is due to begin October 9.
WISN 12 News reporter Nick Bohr says the key question unanswered so far is: Was Weier able to conform her conduct to the law?
This came after a Waukesha County jury reached a verdict in the first Slender Man trial.
In his questioning of Trussoni, Assistant District Attorney Kevin Osborne focused on the fact Weier didn't clearly tell anyone she was afraid of Slender Man's wrath until after the stabbing, reinforcing his comments in opening statements Tuesday that Weier understood that what they were doing was wrong, but that she went along with the plot to preserve her friendship with Geyser.
Both Weier and Geyser were charged with being a party to attempted first-degree intentional homicide.
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