The defense argued that 15-year-old Anissa Weier was mentally ill at the time of the attack.
The verdict allows her to serve time in a mental hospital.
In 2014, Weier and a friend stabbed a classmate 19 times in order to please a horror character named Slender Man.
The victim survived, and was found by a passerby.
All three girls were just 12-years-old at the time.
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