A judge in Waukesha County Circuit Court is expected to send 16-year-old Anissa Weier to a facility for at least three years after she was previously found not guilty by reason of mental disease or defect. Picture: Rick Wood/Milwaukee Journal-Sentinel via AP

Waukesha — One of two Wisconsin girls accused of stabbing a classmate to win favour with a horror character named Slender Man is about to learn how long she will spend in a mental health facility.

Anissa Weier (ah-NEE'-sah WY'-ur) pleaded guilty in August to being a party to attempted second-degree intentional homicide, but claimed she wasn't responsible for her actions because she was mentally ill. In September, a jury agreed.

A Waukesha County Circuit Court judge is expected Thursday to send the 16-year-old to a facility for at least three years.

Morgan Geyser, one of two Wisconsin girls charged with stabbing a classmate, Payton Leutner, in 2014, to impress the fictitious horror character Slender Man, was also found not guilty by reason of mental disease and will be sentenced in February 2018. Picture: Michael Sear/Milwaukee Journal-Sentinel via AP

Payton Leutner (LYT'-ner) survived 19 stab wounds in a Waukesha park in 2014. Leutner, Weier and the other attacker, Morgan Geyser (GY'-zer), were 12 years old at the time.

Geyser also entered a plea and will be sentenced in February.

Read more about the case:

Teen escapes jail for her part in Slender Man knife attack

Slender Man attackers descended into 'madness': lawyer

Associated Press