7 Chilling Revelations from HBO’s New Slenderman Documentary

Shock rippled through a small Wisconsin suburb in 2014 when two 12-year-old girls stabbed a young friend. When their story and motive—they said they did it to please a fictive Internet bogeyman called “Slender Man”—became known more widely, the rest of the world was horrified too.

Morgan Geyser and Anissa Weier confessed to luring their 12-year-old friend Payton “Bella” Leutner into the woods near Waukesha and leaving her to die from her 19 stab wounds, police said. The attack had been planned over the course of months. A new documentary, Beware the Slenderman, revisits the case with lengthy interrogation-room footage of the girls and unprecedented access to both their families. The film, which airs on HBO on Jan. 23, explores how an Internet meme helped turn two tweens into possible murderers.

“It was just immediately obvious that this was a horrifyingly real-life zeitgeist example of how we are adapting,” Irene Taylor Brodsky, who directed the documentary, told TIME. “I’m not saying that every child on the Internet is going to try to kill their friend, but it’s a real jumping off point to talk about all the what-ifs.”

Here are some of the documentary’s most chilling revelations. WARNING: Some of the content below is graphic and may be disturbing.

It all began at a slumber party.

The three girls had just woken up from a birthday slumber party at Geyser’s house, where they ate doughnuts and strawberries for breakfast before bounding out of the house toward a nearby park with a kitchen knife tucked away into one of their jackets.

Geyser and Weier said they believed the murder would serve as a sacrifice, sparing them from the wrath of the feared, faceless figure they thought real. The “Slender Man,” who they knew to be tall and thin and a terror to children, had the ability to harm people at random and afflict kids with nausea, nosebleeds and nightmares.

At least that’s what they learned from CreepyPasta, a website of horror stories where tales of “Slender Man” are bountiful. “I didn’t want to do this. I was afraid of what would happen if I didn’t,” Geyser told a police detective during questioning.

Despite being convinced of the murder’s necessity, the perpetrators feared its execution.

Geyser and Weier initially wanted to convince Leutner to make herself fall asleep in a park bathroom, where they first planned to kill her.

In her one-on-one police questioning, Weier said she wanted her victim to either be asleep or unconscious in order to avoid hearing her scream for her life and looking into her open eyes. “From what I’ve read … it’s easier to kill people when they’re either asleep or unconscious, because when you look into a person’s eyes you can see yourself, and you don’t want to be killing yourself,” she said. “So I asked Bella if she could put herself to sleep. I kind of went like that to her forehead—banged her head up against the concrete. I don’t like screaming. That’s one thing I can’t handle. And then Morgan handed me the knife. Morgan kind of started freaking out a little bit. She said, ‘I can’t do this. I’m too scared. You have to.’ So I had to hug her and calm her down. And then I had to start petting Morgan like she was a cat.”

When their initial plan failed, they resorted to a children’s game as a ruse.

When the first plot failed, the girls decided to play a game of hide-and-seek to drive their victim deeper into the woods. “She was going to hide one place. I was going to hide another, and then Morgan and I were going to be like lionesses chasing down a zebra,” Weier said, according to police footage.

They describe the crime itself in horrific detail.

Both young girls appeared to lose their nerves when the moment came to strike. “Morgan handed me the knife and said, ‘I can’t do it. You know where all the soft spots are.’ And then I gave it back to her and said, ‘You do it. Go ballistic. Go crazy. Make sure she’s down,’” Weier recalled.

Geyser agreed, telling her Leutner: “Don’t be afraid. I’m only a little kitty cat.”

Then Geyser straddled her victim’s legs and whispered in her ear, “I’m so sorry” before beginning to stab her.

The victim didn’t die.

Leutner, who managed to crawl her way out of the woods to find help, survived the vicious attack. Police found her assailants later that day as they were walking toward the Nicolet National Forest, where they believed “Slender Man” lived in a mansion.

The trial is likely to be widely watched and turn on issues of mental health.

Geyser and Weier are slated to head to trials in adult court this year after having pleaded not guilty by reason of mental disease, the Associated Press reported.

According to the documentary, Geyser was later diagnosed with schizophrenia and is on anti-psychotic medication at a state hospital, while Weier is at a county jail for juveniles.

Grisly crime aside, the trial is likely to raise other important legal issues.

Brodsky, who sympathizes with the two young attackers, said the case begs the important question of how the U.S. punishes children who have no prior history of violence.

“We’re a society that sides with victims. I think this victim deserves 110 percent of our understanding and sympathy, but that doesn’t mean we leave the perpetrators in the dust,” she said. “I think there a lot of people living in our world, particularly parents living in America right now, who want to know how they can keep this from happening to their own children. The film is really an opportunity to talk about these things.”

While true crime documentaries have been sprouting streaming services and airwaves, the new film grips viewers with disturbing police video of both young girls callously confessing to the crime.

“They were clearly in a very distressed and disturbing state,” Brodsky said. “It’s not easy to watch how they talk in their interrogations.”