15-year-old Anissa Weier, who admitted to participating in the stabbing of a classmate to please horror character Slender Man when they were only aged 12, will avoid prison after a jury determined Friday that she was mentally ill at the time of the attack.  

The decision means the Wisconsin teen will be sent to a mental hospital rather than prison. A plea agreement calls for her to spend at least three years at a hospital. Before jury deliberations on Friday, Anissa Weier's attorney told the court his client was lonely, depressed and "descended into madness," as he pleaded with a jury to send the girl to a mental hospital rather than prison.

Weier and Morgan Geyser lured classmate Payton Leutner into the woods at a park in Waukesha, a Milwaukee suburb, in 2014. Geyser stabbed Leutner 19 times while Weier urged her on, according to investigators. The girls attacked Payton Leutner after a sleepover in 2014 to prove the fictional boogyman, Slender Man, they believed in was real. All three girls were 12 at the time.

The three girls had spent the night at Geyser's home the night before the attack. Geyser and Weier plotted it overnight and Geyser carried the knife in her waistband as they marched in to the forest.  After the stabbing, the pair ran away, leaving Payton for dead. Leutner barely survived her wounds and managed to crawl into the path of a passing cyclist who called for help. The girl was then taken to hospital where doctors said she was a "millimeter from death". She returned to school in the fall of 2014.  

After the stabbing in 2014, the pair both told police about Slender Man and described their fear that they would have been punished by him had they not attacked Payton. Both Weier and Geyser told detectives they felt they had to kill Leutner to become Slender Man's "proxies," or servants, and protect their families from the demon's wrath.

Weier, now 15, pleaded guilty to attempted second-degree intentional homicide in a deal with prosecutors in August. But she claims she was mentally ill during the attack and not responsible for her actions. Their was a trial to assess Weier's mental competency and to determine whether she should be sentenced to a prison sentence or a psychiatric facility. If she is convicted as a criminal she would have spent 10 years or more in behind bars.

On Wednesday Melissa Westendorf who was appointed by a judge to evaluate Weier after her insanity plea, testified for the teen.  Westendorf testified that she believed Weier suffered from a shared delusional disorder that left her unable to conform her conduct to the law when she and Geyser tried to kill their friend Leutner in 2014.

Weier's attorney, Joseph Smith Jr., asked why Weier, a good student, did not recognize that the belief in Slender Man and his powers to kill them or their families was a delusion and Westendorf answered: "First of all, she was 12", adding that Weier was influenced by a website focused on imaginary killers and boogeymen.

"If adults have trouble distinguishing fake news, 12-year-olds will, because their brains can't yet discern or analyze as well," Westendorf added.

A jury heard three days of testimony from psychologists and detectives. The jury began deliberations Friday morning. Weier's attorney, Maura McMahon, said during closing arguments that Weier was lonely and depressed after her parents divorced and that she latched onto Geyser. Morgan Geyser also admits her role in the stabbing but has pleaded not guilty to attempted murder by way of insanity. Her mental competency trial will begin in October.

McMahon said Weier and Geyser became obsessed with Slender Man, developing a condition called shared delusional disorder. Weier believed Slender Man could read her mind as well as teleport and would kill her or her family if she talked about him, she said.

McMahon said" "This sounds crazy, because it is. This was a real being to this child and she needed to protect those around her. At 12-years-old, she had no way to protect herself from (Slender Man) except for Morgan's advice and they swirled down into madness together."

Waukesha County Deputy District Attorney Ted Szczupakiewicz countered during his closings that the stabbing was calculated. He said the girls had planned the attack for at least four months. He asked jurors to consider why if the girls were so afraid of Slender Man they waited so long to attack Leutner. He also pointed out that Weier told a detective she wasn't frightened of Slender Man until after the attack, when Geyser told her she had made a deal with the monster that he would spare their families if they killed Leutner.

"It comes down to did she have to or did she want to?" Szczupakiewicz said. "It wasn't kill or be killed. It was a choice and she needs to be held criminally responsible."

Wisconsin law requires only 10 of 12 jurors to render a verdict on whether a criminal defendant wasn't responsible for her actions due to a mental condition. The jury spent all day Friday deliberating and concluded late Friday night. 

On Monday, prosecutors rejected the argument and told jurors both girls were aware of what they were doing when they turned on Payton in the forest. 

"They knew this was wrong. They understood what they were doing was wrong," attorney Kevin Osborne said.