Bruce Vielmetti

Milwaukee Journal Sentinel

Anissa Weier plans to enter a plea of not guilty by reason of mental disease or defect in the Slender Man stabbing case next week.

Weier's co-defendant, Morgan Geyser, had already entered such a plea, often referred to as NGI, at a hearing last month.

At the time, prosecutors pressed the judge to inquire whether Weier might go the same route, since it requires the court to appoint doctors to examine the defendant and report whether their condition would support the plea. One of Weier's attorneys, Maura McMahon, said the defense had not yet decided.

But in a pleading filed this week, McMahon indicated that Weier now wants to change her not guilty plea to the insanity plea. A hearing was set for Sept. 9 for that change of plea.

Weier and Geyser, both 14, were 12 when prosecutors say they plotted and attempted to kill their sixth-grade classmate and friend Payton Leutner in May 2014.

Insanity plea entered in Slender Man case

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Leutner was stabbed 19 times and left for dead in a Waukesha park the morning after a sleepover party for Geyser's 12th birthday, but managed to crawl near a path, where she was found by a passing bicyclist.

Both defendants later told police they were trying to either impress or avoid the wrath of Slender Man, a fictional internet boogeyman the girls said they believed would harm them or their families if they didn't kill their friend.

They remain charged as adults with attempted first-degree intentional homicide, after efforts to have their cases transferred to juvenile court were denied.

A defendant entering an NGI plea contends that at the time she committed a crime, a mental disease or defect prevented her from appreciating the wrongfulness of her action, or from conforming her conduct to the law.

Someone found not guilty by reason of mental disease or defect is typically committed to a state mental hospital for treatment.

Geyser, who has been diagnosed with early onset schizophrenia, has already spent months at Winnebago Mental Health Institute under a separate civil commitment.

During earlier hearings, mental health experts testified that Weier was competent to proceed with the case, but had not done more extensive evaluations required for an NGI review. She remains at a West Bend juvenile detention center.

No trial date has been set in the case.