Anissa Weier is brought into court for a hearing last month. On Thursday, her attorney said she would plead not guilty by reason of mental disease or defect in the Slender Man case. She and Morgan Geyser are accused of stabbing another girl. (Photo: Michael Sears / 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. 
 
 ARCHIVE: Slender Man case coverage 
 
 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. 
 
 Read or Share this story: http://on.jsonl.in/2civi3X ||||| CLOSE Case developments at a glance. Bruce Vielmetti / Milwaukee Journal Sentinel 
 
 Anissa Weier, 15, waits for her trial to start before Waukesha County Circuit Judge Michael Bohren in Waukesha on Tuesday. (Photo: Rick Wood / Milwaukee Journal Sentinel) 
 
 WAUKESHA – A girl who helped a friend try to kill their classmate shouldn't be held criminally responsible for the crime because she believed at the time that she and or her family would be killed by the internet character Slender Man if she didn't act, her lawyer told a jury Tuesday. 
 
 "How could she have believed this?" Joseph Smith Jr. asked rhetorically. He said three psychological experts would explain how his client, Anissa Weier, and her co-defendant, Morgan Geyser, shared a delusion and lost touch with reality. 
 
 During his opening statement, Smith played segments of Weier's disturbing interrogation during which she described Slender Man and his purported powers to a police detective who had never heard of the character. 
 
 "He could easily kill my whole family in three seconds," she tells the detective. 
 
 A prosecutor said the state won't dispute the experts but said the real issue is why Weier went along with the attack. 
 
 "She wanted to preserve her one and only friendship" with Geyser, Assistant District Attorney Kevin Osborne said, and she knew the stabbing was wrong because she twice declined to do it herself as originally planned. 
 
 Instead, Osborne said, Weier told Geyser to do it. "Go berserk," was the command, he said. 
 
 The opening statements began what is expected to be a weeklong trial on whether Weier should be sent to prison or committed to a state mental hospital. 
 
 RELATED: Jury to decide whether Slender Man attacker, now 15, goes to mental hospital or prison 
 
 ARCHIVES: The Slender Man stabbing case 
 
 Weier, 15, was 12 in May 2014 when she and Geyser carried out a plan to kill their sixth-grade classmate, Payton Leutner, after a sleepover. The victim survived multiple stabbings, and Weier and Geyser were charged as adults with attempted first-degree intentional homicide. 
 
 Weier recently agreed to plead guilty to attempted second-degree intentional homicide, and stand trial only on whether she should be held criminally responsible. Both girls have pleaded not guilty by reason of mental disease or defect. 
 
 Sixteen jurors began hearing the case Tuesday after spending the night sequestered at a hotel. Weier's attorneys have the burden of proving that at the time of the crime, she suffered from a mental disease or defect that prevented her from understanding her conduct was wrong or from conforming her actions to the law. 
 
 CLOSE The insanity defense allows some defendants to avoid legal responsibility for their acts. Bruce Vielmetti / Milwaukee Journal Sentinel 
 
 Civil court standards apply, so only 10 of the jurors must be persuaded to a reasonable certainty, as opposed to the criminal standard of unanimity that the evidence is beyond a reasonable doubt. 
 
 Father testifies 
 
 The defense's first witness was Weier's father, William Weier. He described her time in school and the trying situations in their home life in 2013 and 2014. But through all that, he said, he never saw anything to suggest she needed mental health care. 
 
 William Weier, father of Anissa Weier, testifies during her trial that it was a stressful time for the family during his divorce proceeding, but that he didn't notice any unusual behavior with Anissa. (Photo: Rick Wood / Milwaukee Journal Sentinel) 
 
 "In my opinion, she was a normal child," he said. 
 
 He said his feedback about his daughter in fourth and fifth grade was that she was a good student, not disruptive, and very interested in reading nonfiction, mostly about animals. 
 
 At the same time, he and Weier's mother were going through a divorce. He said they told Anissa and her younger brother that they were in no way a cause of the split and that they could talk with counselors at school or elsewhere if they wanted to. 
 
 William Weier said Anissa had attended Prairie Elementary School, not her zoned school, and she was upset she would be going to a different middle school than her classmates. 
 
 But he said she liked the Flight Academy program for advanced students at Horning Middle School, where students from all three grades could work independently or in groups for more specialized learning and seemed to settle in there. 
 
 She used to ride the bus with three other girls until he texted them back on her phone to stop bothering Anissa while she was doing her homework. After that, he said, they seemed to shun her. Soon after, she met Geyser on the same bus. 
 
 He said Geyser did some unusual things, like come to the bus stop wearing a mask, or Vulcan ears, but he thought it more goofy than disturbing. He said Geyser's parents, who lived in the same condominium complex, introduced themselves and seemed glad Morgan had a nearby friend. 
 
 Weier said his attention during Anissa's year in sixth grade was largely focused on supporting her older step-siblings because their mother, his first wife, was seriously ill and died in February 2014. He was helping those children move from Milwaukee to his home in Waukesha the day of the stabbing. 
 
 'Don't worry, it's not you' 
 
 A former Flight Academy classmate of Weier's who shared an interest in Slender Man said one day Weier told her she'd discovered how to become a proxy — kill a friend. 
 
 "Don't worry, it's not you," the girl, identified in court by her initials, K.N., testified Weier told her. 
 
 K.N. said she told her mother and twin what Weier had said. The twin testified that she then told a counselor at school about Weier's remark. 
 
 Waukesha police Det. Thomas Casey, the lead investigator in the case and the person who interrogated Geyser, testified most of the afternoon. Defense attorney Maura McMahon used Casey to introduce disturbing drawings and slogans found in Geyser's school notebooks, things like NEVER ALONE, SAFER DEAD, HE CANNOT BE HARMED, usually accompanied by circles with Xs for eyes, a Slender Man symbol. 
 
 Waukesha Police Department Detective Thomas Casey, lead investigator in the Slender Man case, testifies about notebooks found in the school lockers of Anissa Weier and Morgan Geyser. (Photo: Rick Wood / Milwaukee Journal Sentinel) 
 
 The testimony seemed aimed at suggesting Geyser, who has been diagnosed with and is being treated for schizophrenia, was the catalyst that set off Weier's lesser condition, and caused the two, together, to take such extreme action in furtherance of their shared delusion. 
 
 The defense also introduced pictures of the bag the girls had when arrested and the ridiculously inadequate provisions they had packed for an expected 330-mile walk to Slender Man's mansion in the Nicolet National Forest — some snack bars, notebooks, two water bottles, and pictures of their families, and the kitchen knife they are accused of having used on Leutner. 
 
 On cross-examination, prosecutors introduced a note Weier wrote on her phone to her family, that reads like a last will and testament, and a goodbye. Deputy District Attorney Ted Szczupakiewicz also had Casey talk about email exchanges between Weier and Geyser that showed the secretive nature of their plot. Weier even created a new email account a few weeks before the attack so she could talk 'freely about that thing." 
 
 At one point, the judge took a break because one juror's hearing aid had failed. The defense was concerned how much testimony the woman might have missed. The trial resumed after new batteries were found for the hearing device. 
 
 The jurors, eight men and eight women, range in age from 20 to 80, according to court officials, who declined to provide any other description of the court-ordered anonymous jury. 
 
 The trial continues Wednesday. 
 
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– Both girls in Wisconsin's disturbing "Slender Man" case will now be pleading not guilty by reason of insanity. Attorneys for 14-year-old Anissa Weier signaled this week that they want to change her plea in the attempted murder case from not guilty to "NGI," meaning that she's likely to be committed to a state mental hospital for treatment if found not guilty, reports the Milwaukee Journal Sentinel reports. Co-accused Morgan Geyser, who is also 14, has been diagnosed with early onset schizophrenia and has already entered an NGI plea. Both girls were 12 years old in 2014 when they allegedly stabbed a 12-year-old classmate 19 times and left her for dead in what they told investigators was an attempt to please the online horror character "Slender Man." In July, an appeals court ruled that they should be tried as adults for the attempted killing. WISN reports that lawyers for both girls are seeking to have the case heard by jurors from outside Waukesha County, where the attack took place, because of heavy media coverage.