The jurors had to decide whether Anissa Weier was mentally ill when she and Morgan Geyser stabbed a classmate, Payton Leutner, 19 times in May 2014 in order to win the favor of the fictional character Slender Man.
After nearly three more hours the jury emerged with the verdict which, per Weier's plea agreement means she will be placed in a mental institution until at least 2020, WDJT reported.
While Weier plead guilty to her charges, her attorney claimed she was mentally ill at the time of the attack.
Her trial is set to start in October.
Prosecutors contend Weier knew what she was doing was wrong, but that she went along with the plot to preserve her friendship with Geyser.
"The potential release of an assailant that methodically planned and executed an attack in which our little girl was stabbed 19 times puts the community and our family at risk", the statement continued.
Jurors determined Anissa Weier was mentally ill during the stabbing in 2014. "At 12 years old, she had no way to protect herself from this except to follow Morgan's advice, and they swirled down into madness together".
Waukesha Police Detective Shelly Fisher's testimony Wednesday came in the trial of Anissa Weier.
Weier told investigators she and Geyser believed they had to kill Leutner or else Slender Man would kill them and their families. While Weier was dealing with her parents' divorce, teachers began noticing symptoms of depression, he said. She will be sent to a mental hospital for at least three years.
Slender Man first appeared on the internet in 2009.
Van Rybroek examined Weier previous year.
Weier's lawyer Maura McMahon said, "Life is better for children when adults around them are in communication with each other". She was not accessible a short time later, but rather McMahon said she was diminished and cried following the decision. Leutner told Fisher her mother tried to assure her it was all fiction, but she also told the detective that she still was uncertain and wanted to believe Geyser even after she stabbed her, the Milwaukee Journal Sentinel reported.
Geyser, who did the stabbing, will go on trial next month.
Leutner managed to survive the attack.
Both Weier and Geyser were charged with being a party to attempted first-degree intentional homicide.
Weier pled guilty as part of a plea agreement in August to attempted second-degree homicide as a party to a crime with use of a deadly weapon, court records show.