MILWAUKEE — One of 10 jurors who decided a Wisconsin girl was mentally ill when she helped stab a classmate to please the fictional horror character Slender Man said Monday the most powerful evidence was testimony by medical experts.

Anissa Weier, now 15, will go to a mental hospital rather than prison when she is later sentenced by a judge. Weier and a co-defendant, Morgan Geyser, admitted to stabbing classmate Payton Leutner in a Waukesha park in 2014, but Weier argued she was mentally ill and not responsible for her actions.

All three girls were 12. Leutner survived after crawling for help and being found by a passing bicyclist.

The jury heard a week’s testimony and deliberated most of a day before reaching a decision late Friday night. The juror told WTMJ-TV that medical testimony about Weier’s mental stability was compelling, especially from those doctors who were appointed by the court. WTMJ withheld the juror’s identity because it said threatening comments had been posted about the jury on social media.

“The first two doctors, we thought they were completely unbiased. They weren’t hired by the defense, they weren’t hired by the prosecution, they were appointed by the court,” the juror said.

Two forensic psychologists testified that Weier and Geyser had a shared delusion that Slender Man — an internet creation said to lurk near forests and target children — was real and could kill their families. A defense expert gave similar testimony.

Prosecutors argued that Weier knew what she was doing was wrong, but she went along with the stabling plot to preserve her friendship with Geyser.

The juror said he also thought others who didn’t detect a problem before the attack were partly responsible.

“I do think the parents failed them, I think the teachers failed them, I think in general society failed these girls,” the juror said.

Weier’s father, William Weier, testified early in the trial that his daughter went through trying times in grade school as her parents divorced, but he described her as “a normal child.”

A plea agreement reached before trial called for Weier to spend at least three years in a mental hospital before seeking release if she was found mentally ill. A presentencing investigation is due Oct. 2.

Geyser’s trial is due to begin Oct. 9. She has also pleaded not guilty by reason of mental disease.

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