What would cause two 12-year-old girls to stab their classmate 19 times?

It's a macabre yet very real question that documentary maker Irene Brodsky asked when the incident in Wisconsin made headlines across America.

The answer? The girls claimed it was a sacrifice to please a fictional ghoul from the internet called Slender Man.

As we anticipate the release of an HBO documentary series on the incident, we take a look at everything you need to know about the case. 

If you're already not on the edge of your seat, you will be. 

What exactly happened?

On Friday 30 May 2014, Morgan Geyser invited her friends Anissa Weier and Sally Brown* over for a sleepover at her home in Waukesha, Wisconsin, to celebrate her 12th birthday. In the morning Morgan asked her mum if they could go outside to play, reports New York Magazine. As the girls walked to a nearby park, Morgan showed Anissa that she'd taken a knife from the kitchen.

Anissa suggested they go play hide-and-seek in the woods on the edge of the park and when Morgan started to count, she tried to tackle Sally to the ground, but couldn’t hold her down. It was when Anissa started to walk away that she stopped and shouted “Kitty now. Go ballistic, go crazy.”

“Don’t be afraid, I’m only a little kitty cat,” Morgan replied, and then pushed Sally over and stabbed her 19 times, in her arms, legs, stomach, liver, pancreas and barely missed a major artery near her heart. “Stabby stab stab stab” is how Morgan recalled it during her interview with police. “It didn’t feel like anything. It was, like, air.” 

Morgan and Anissa fled the park and washed themselves in the bathroom at a local Walmart and wandered around Waukesha, while Sally crawled onto a nearby road and was taken to hospital by a passing cyclist. 

During their interviews with detectives that day, Anissa and Morgan, said they were trying to kill Sally to please a mythical horror creature on the internet named Slender Man. Anissa ­explained their idea was to become proxies, or ­puppets, of Slender Man through a sacrifice of blood. Later, they said they would go to live with him in a mansion in the forest, morphing into mini-­monsters, not unlike the way humans who’ve been bitten by vampires are said to become vampires themselves.

Both girls were charged with first degree attempted murder.

*we've chosen to change the victim's name to protect her identity.   

Photo: Waukesha Police Department

Who is the Slender Man?

On 8 June 2009, a member of the online forum Something Awful challenged members to "create paranormal images through Photoshop". Members instantly began creating a variety of ghostly and spine-chilling images. But on the second day, things took a turn when one forum member posted two photos (one pictured below) of children with a faceless, suited and tall man with long tentacle-like limbs looming in the background.

“Almost immediately, an obsessive interest in the Slender Man took over the forum discussions,” writes assistant professor Shira Chess at the University of Georgia, in a journal report about the phenomenon. A communally-created, horror villain was born.

New photographs, drawings, short fiction and even wood cuttings continued to develop the idea of the Slender Man in different places and times. The story was even turned into a YouTube series called Marble Hornets, which sparked many more videos and interpretations.

The Slender Man has become a 21st century boogeyman whose influence and power frighteningly surpassed the realms of the internet and into an attempted homicide case in Wisconsin.

How did Slender Man go from fiction to fact?

Like all 12-year-olds, Morgan and Anissa fantasised about their favourite fictional characters. The only distinct difference was that they were obsessed with villains in popular culture and supernatural evils in internet fiction.

Morgan, in particular, affectionately called Voldemort from J.K. Rowling's Harry Potter series, “Voldie,” as if he were a pet.

The pair, whose friendship blossomed on the ride to and from school, had a shared a fondness for Slender Man among other mythical characters. Just two months before the stabbing, Morgan's father Matt posted a drawing Morgan made of Slender Man: “Only Mogo draws Slender Man in crayon on a napkin when we are out to dinner”.

It's unclear when the internet villain transcended from fantasy to reality for the girls, but it certainly became a preoccupation of theirs. Police found maps and checklists referring to the stabbing and days before the incident Anissa wrote a will, bequeathing all her possessions to her parents, as well as an adieu: “This is my final wish to those who care, do not grieve my absence, but remember me for who I was. I love and ­cherish you all and wouldn’t do you harm,” New York Magazine reports.

“We were probably going to be spending the rest of our lives [with Slender],” Anissa, later told police, “and I didn’t want to forget my family.”

Morgan told police that it was “difficult” to not tell anyone about their plotting and that she went through with the stabbing in part because “I didn’t want to make Anissa mad. It’s hard enough to make friends. I don’t want to lose someone over something like this.”

What happened after the stabbing?

For the last 14-months Anissa and Morgan have been living in a juvenile-detention facility in Washington County. They are intentionally kept apart, living in separate living quarters, attending separate classrooms.

An effort to have them trialed as children failed in August 2015, after the judge cited the particularly vicious nature of the crime and ruled to maintain their status as adults. Psychologists agree that by the age of eight, and certainly by 12, most children are as able as adults to sort out what’s real from what is not.

During police questioning on the day of the stabbing, Anissa admitted that the Slender Man “does not exist. He is a work of fiction.”

Morgan, however, who suffers from schizophrenia - a brain disorder that causes people to abnormally interpret reality - had a greater attachment to the fabricated tale. But she too admitted in her interview that the attack on Sally was “probably wrong. I honestly don’t know why we did this.”

What can we expect from the TV documentary?

As soon as American documentary maker Irene Brodsky heard about the stabbing incident, she travelled to Wisconsin and quickly gained the trust of Morgan and Anissa's parents. 

She followed the lives of the two girls for over 18 months, documenting how and why they committed the crime and delving into how it relates to the morality of modern media and its impact on young minds. 

"In no way is this a whodunit. It's a whydunit. It's a howdunit. How could this have happened? And the answers are very complex," Brodsky says. 

"The film goes into the minds of the perpetrators, but I also think it goes into the mind of us as a society…. Slenderman appeals to us for many reasons. The facelessness of him means he can be whatever we need him to be. He's not just a man that preys on children. Some children see him as a guardian angel. Some kids see him as a protector. These girls, they really believed in his power." 

The documentary Beware of the Slenderman (poster below) screened three times at SXSW Festival this month and received rave reviews (8.9/10 on IMDb).

It's unknown when Beware the Slenderman will be out in the UK but it will release on HBO later this year.