After a full day of deliberations, which lasted until late Friday night, a jury in Waukesha County found that Anissa Weier was mentally ill when she and a friend attacked another girl, leaving her to die in the woods. All three girls were 12 years old at the time.
"There's no walking away for Anissa, no loophole", McMahon said.
Anissa Weier, now 15, had pleaded guilty in August to being a party to attempted second-degree homicide.
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.
"Anissa's broken mind caused her to lose touch with reality", Smith said, according to the AP.
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.
Michael Caldwell, a psychologist hired by Weier's attorneys to evaluate her in 2014, also testified Thursday.
YOUTUBE Online horror character Slender Man
Prosecutors contend Weier knew what she was doing was wrong, but that she went along with the plot to preserve her friendship with Geyser.
Weier will now be admitted to a mental institution. McMahon said before the inconsistent verdict came in Weier "was shaking so hard, she was shaking me". "She knew the difference between right and wrong, but even so she could not stop herself at the moment it mattered most", he said.
The Slender Man is a fictional supernatural character.
Authorities say the two were motivated by a fictional horror character called Slender Man to stab Payton Leutner 19 times in a Waukesha park. "It was a choice and she needs to be held criminally responsible", the Fox News reported.
Wisconsin law requires only 10 of 12 jurors to render a verdict on whether a criminal defendant wasn't responsible for her actions due to a mental condition.
Weier said she had second thoughts when she and Geyser began their 300-mile walk to Slender Man's supposed mansion in Nicolet National Forest, but Geyser encouraged her to continue by saying she'd made a deal with Slender Man that would result in the deaths of their families if the girls didn't join him.
Maura McMahon, Weier's attorney, said she trusted the case uncovered that kids might be taking care of mental issues lost on grown-ups who have turned out to be excessively occupied with their own particular lives, making it impossible to focus and assets proliferate to help them.