Slender Man
A now-notorious stabbing was dubbed the “Slender Man” case because Geyser reportedly believed she was able to communicate with the fictional, supernatural, blank-faced figure shown here. Getty Images/Dan Kitwood

One of the defendants, a Wisconsin teenager, in the notorious “Slender Man” stabbing case, who was accused of stabbing a classmate in 2014 in order to win favor with a fictional internet character named “Slender Man,” has been sentenced Thursday to 40 years in a psychiatric institution instead of serving jail time.

Morgan Geyser, 15, who pleaded guilty to the stabbing in October, was ordered to receive treatment for schizophrenia by Circuit Judge Michael Bohren, CBS News reported.

"What we can't forget is this was an attempted murder," Bohren said. He said he believed Geyser remained at a risk to hurt herself and others and called it "an issue of community protection."

ABC News obtained the footage from Geyser’s police interrogation shortly after the stabbing. In the video, she admitted that the goal was to kill their victim in the name of “Slender Man,” in hopes of protecting their families from his spectral wrath. The footage is expected to be broadcast Friday on the next episode of ABC’s 20/20, which will focus on the case.

The attack took place on May 31, 2014, when Geyser and her classmate Anissa Weier lured one of their classmates Payton Leutner to a park in Waukesha, Wisconsin, and stabbed her 19 times. The victim was discovered on the side of the road by passersby. The attempted murder came to be known as the “Slender Man Stabbing” as the two students claimed that they were influenced by the internet and video game fictional character "The Slender Man."

The video of the interrogation obtained by ABC showed Geyser only a few hours after the attack.

"What were you trying to do with her when you stabbed her?" a detective is heard asking.

"Kill her," Geyser responded to that. "I might as well just say it. We were trying to kill her."

All three girls were 12-years-old at the time of the incident.

Geyser’s mother, Angie Geyser said she was in disbelief that her daughter could have planned to hurt someone when the detectives told her about it.

“I couldn't believe what I was hearing,” Angie Geyser told ABC News’ “20/20.” “I never would have imagined that my daughter was capable of hurting another person.”

After reviewing the tape, Angie told ABC’s 20/20 that her daughter never acted the way she did during the interrogation.

“The interrogation is very difficult to watch because that’s not my daughter saying those things. That’s not the way that she speaks, that’s not the way that she acts,” Angie said.

"(She) appears to have no remorse for what’s happened and she just talks about it in such a flat manner," Angie said of her daughter.

Both Angie and Weier’s mother said that they never saw any warning signs from their daughters that they would harm someone else.

“They [Morgan and Payton] would sit up in Morgan's room and they would do each other's nails, and they would laugh, and make a mess," Angie said. "They were just typical girls.”

Angie said her daughter had spoken to her about Slender Man and talked about it with her.

“She would show us some of the pictures, and she would read us some of the stories, and while some of the subject matter was a little dark, I wasn’t concerned,” Angie Geyser said. “When I was Morgan's age, I was reading Stephen King novels. I remember being 11 years old and riding home from the library with [the book] 'IT' under my arm. And that's a very scary and dark story, so I just thought it was normal for a child of middle school age to be interested in scary stories.”

However, Weier’s mother, Kristi Weier, said her daughter “never mentioned anything to me about her belief in Slender Man.”

Leutner managed to survive the attack, Geyser was charged with first-degree attempted murder, while Weier was charged with second-degree attempted murder, to which both pleaded guilty.