WAUKESHA, Wis. -- A jury has been selected in southeastern Wisconsin to decide the mental competency of a girl accused of stabbing a classmate in 2014 to please a fictional horror character known as Slender Man.

Twelve jurors and four alternates were selected Monday in the case of 15-year-old Anissa Weier. Opening statements are scheduled Tuesday.

Weier has pleaded not guilty due to mental illness in a stabbing that nearly took the life of classmate Payton Leutner at a Waukesha park when the girls were 12. A second classmate, Morgan Geyser, will go on trial later.

Several potential jurors were dismissed because they said they don't believe in the insanity plea.

A plea deal calls for Weier to be committed to a mental hospital for at least three years if she's found to have been mentally ill.