Prosecutors allege that Weier and her friend, Morgan Geyser, lured their classmate Payton Leutner into a Waukesha park in May 2014 and stabbed her 19 times.
Smith said during opening statements Tuesday that Weier believed she and Geyser had to kill Leutner in order to become proxies of Slender Man, and that if they didn't, the fictional character would harm her family. All three girls were 12 at the time of the attack.
Anissa and Morgan were both charged with first degree intentional murder and are being tried as adults.
The ongoing trial is to assess her mental competency and will determine whether she should be sentenced to a prison sentence or if she will be sent to a psychiatric facility. The state recommended a 10-year prison sentence, although the judge can sentence her to up to 25 years.
Jury selection began Monday in the trial of one of two United States teens accused of stabbing a friend to appease a fictitious internet character, in a case that sparked a TV documentary "Beware the Slender man".
Fifteen-year-old Anissa Weier has admitted to participating in the attack on Payton Leutner at a park in Waukesha in 2014.
"Anissa's broken mind caused her to lose touch with reality", Smith told jurors.
The defense attorney for a Wisconsin girl accused of helping stab a classmate to please the fictional horror character Slender Man says she was under the "command and control" of a delusional disorder.
Weier and Geyser are seen above in their mugshots in April 2014. Psychologists have testified that she suffered from persistent depression and a delusional disorder linked to schizotypy, a diminished ability to separate reality from fantasy. Weier and Geyser were arrested later that day while walking to meet Slenderman in a northern Wisconsin forest.
They signed confessions to that effect which their parents and attorneys are now begging to be thrown out on the basis that neither understood properly what they were confessing to at the time.
Several jurors on Monday expressed skepticism of an insanity defense.
"They knew this was wrong", Osborne said. The victim suffered serious injury and the other girls clearly needed guidance that they didn't get, she said.
There were 150 potential jurors in the pool yesterday and narrowing them down took about six-and-a-half hours. Geyser is to stand trial later this year.
The three girls had spent the night at Geyser's home the night before the attack. He said she carried out the stabbing not out of fear, but to preserve her friendship with Geyser.
Osborne also said Weier told investigators she didn't believe her family was threatened by Slender Man until Geyser informed her of that possibility after the two girls had already carried out the stabbing. The jury will decide whether she was indeed impaired. Her attorney, Maura McMahon, didn't return a voicemail.
The Slender Man is a fictional character that originated in an online art contest in 2009, before becoming a popular meme.
The character is said to have creepily long, tentacle-like arms, which can be extended to capture prey.
It is also said to be able to create distortions in photographs and teleport.
It improved, as 34 investors sold Gigamon Inc shares while 80 reduced holdings. 84 funds opened positions while 175 raised stakes. On average, equities research analysts forecast that McDonald's Corporation will post $6.55 EPS for the current fiscal year.
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There are now 3 hurricanes in the Atlantic
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Michael Owen: Manchester United defence are a worry
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