WAUKESHA, Wis. - The attorney for one of two Wisconsin girls accused of trying to kill a classmate to please a fictional horror character called Slender Man is challenging prosecutors' use of his client's confession.

In a motion filed in Waukesha County Circuit Court, the attorney for Morgan Geyser argues she was too young and impressionable and had not been treated for schizophrenia when she talked to investigators on camera about the stabbing of classmate Payton Leutner in 2014.

According to prosecutors, Morgan and Anissa Weier stabbed Payton 19 times in a Waukesha park following a sleepover, then left her and planned to walk hundreds of miles north to meet Slender Man in a forest. Payton was able to crawl out of the woods in the park to a path where she was found by a bicyclist.

Geyser and Weier have pleaded not guilty by reason of mental disease to attempted homicide charges in adult court.

The motion challenging the confession states a defendant's "age, education, intelligence, physical or emotional condition, and prior experience with law enforcement" should be taken into account, as well as pressures imposed by law enforcement officers during an interrogation.

"All of those factors cut against the statement being voluntary," Morgan's attorney Anthony Cotton said Wednesday.

The detective who interrogated Morgan consistently minimized the seriousness of her arrest by asking her questions about school and mischaracterized the purpose of his questions, according to Cotton's motion.

Morgan was not diagnosed until months later with early-onset schizophrenia. But Cotton argues it doesn't take a doctor to realize the existence of a mental health issue at the time she was interrogated.

"She's talking about being a follower of a fictional character. Does this person appear to be suffering from delusional beliefs or not?" Cotton said.

During the nearly five-hour interrogation, Morgan asked Waukesha Detective Thomas Casey if she will regret giving him information. The defence says Casey responded with a problematic answer.

"You know, this information will be used to try to get you some help so you don't have to hear those screams and don't have to worry about hurting anybody anymore," Casey responded. Morgan answered, "OK."

Casey also told Morgan he didn't think she would go to prison.

Cotton said the detective's statements amounted to false promises because Morgan believed her answers would be used to help her, not send her to prison.

The defence also argues that Miranda rights read to Morgan were flawed because the arresting officer told her she had to answer yes or no to understanding the warning, when in fact she could have remained silent.

Both defendants have moved to have separate trials and have asked that a jury be drawn from outside Waukesha County.

Waukesha County Circuit Judge Michael Bohren is scheduled to consider the motions at a hearing next month.

News from © The Associated Press, 2016