WAUKESHA, Wis. - One of two Wisconsin girls who tried to kill a classmate with a knife to appease fictional horror character Slender Man is expected to find out Thursday how long she'll spend in a mental institution for the attack.

Prosecutors want 15-year-old Morgan Geyser to spend the maximum 40 years in a mental hospital for stabbing Payton Leutner in suburban Milwaukee in 2014. But Geyser's attorneys will make their own case for what they believe is best for Geyser during a daylong hearing in Waukesha County Circuit Court.

The hearing will include victim impact statements and doctors who have evaluated Geyser. She pleaded guilty to attempted first-degree intentional homicide in October in a deal with prosecutors to avoid prison.

Geyser and Anissa Weier lured Leutner into the woods in a park and repeatedly stabbed her with a kitchen knife. Geyser stabbed Leutner 19 times while Weier urged her on, according to investigators. Leutner was left for dead but she crawled out of the woods and got help from a passing bicyclist. She and her attackers were all 12 at the time.

Weier was sentenced to 25 years in a mental hospital in December. She had pleaded guilty in August to being a party to attempted second-degree intentional homicide, but she claimed she wasn't responsible for her actions because she was mentally ill. In September, a jury agreed.

Geyser's attorneys have argued in court documents that she suffers from schizophrenia and psychotic spectrum disorder, making her prone to delusions and paranoid beliefs.

A psychiatrist hired by her attorneys previously testified that Geyser believed she could communicate telepathically with Slender Man and could see and hear other fictional characters, including unicorns and characters from the Harry Potter and Teenage Mutant Ninja Turtles series. She also believed she had "Vulcan mind control."

Slender Man started with an online post in 2009, as a mysterious spectre whose image people edit into everyday scenes of children at play. He is typically depicted as a spidery figure in a black suit with a featureless white face.