A 12-year-old seemed 'unusually calm' as she sat covered in blood and confessed to trying to kill her friend, a detective has said.
Morgan Geyser and Anissa Weier lured their pal Payton Leutner to a sleepover and then an outdoor play session on May 30, 2014.
Motivated by a fear for the mythical character Slender Man, the two 12-year-olds then stabbed their friend 19 times, leaving her for dead as they fled the scene.
Thankfully Payton survived the vicious attack, although she has been left with lifelong physical and emotional scars.
More than five years on from the attack and as Morgan and Anissa serve their 40 years and 25 years prison sentences respectively, one of the first police officers on the case has recalled questioning the girls.
Detective Tom Casey sat down with the friends at the Wisconsin police station shortly after the stabbing.
"I mean, she's at a police department and she's covered in blood and…[it's as if] this is like a normal day for her," he said of Morgan in an ABC 20/20 programme.
When the detective asked the girls whether Payton was dead or alive, they didn't seem concerned about her condition.
"I might as well just say it. We were trying to kill her," Morgan said, Detective Casey recalled.
"I figured that I'd get into trouble eventually, though, because mommy always says that whatever you do catches up to you eventually, and it did."
Anissa went on to tell Detective Michelle Trussoni how she had become afraid of Slender Man - a tall, suited character without a face - through a website that collects horror stories.
She became convinced the ghostly figure was real after seeing him, she believed, while on a bus ride.
The girl told Detective Trussoni that it was "really scary" to think that Slender Man could kill her whole family in three seconds.
All pupils at the same school, Morgan and Anissa thought they had come up with the plan for the perfect murder.
They had invited Payton to a sleepover and originally planned to stab her that night - but couldn't go through with it.
Instead, the following day, they grabbed a kitchen knife from home and set off for the woods.
After playing for some time, they started a game of hide and seek and when Payton was crouching down, they pounced.
She was pinned down and as she stabbed her friend multiple times in a frenzied attack, Geyser shouted: "Don’t be afraid. I’m only a little kitty cat."
Before the girls fled the scene they told Payton they were going to get help.
In her police interview Anissa said: "So, we told her we we’re gonna get help, but we really weren’t. We were going to run and let her - pass away."
After she was found and rushed to hospital by ambulance, Payton told police the shocking story of what had happened to her.
Horrified detectives then launched a search to find the two missing girls.
Morgan and Anissa had set off to the Nicolet National Forest, where they believed Slender Man's castle was located.
The girls were found several miles away - they had taken some fruit sticks with them in case they got hungry and needed the energy.
When investigators scoured the girls' belongings, uncovering a cache of drawings of the spooky character in Morgan's locker and her bedroom.
She also had mutilated dolls in her possession.
Psychiatrists later diagnosed her with early onset schizophrenia.
Both girls are now 17-years-old and are being held at the Winnebago Mental Health Institute in Wisconsin.
They must serve at least three years in locked confinement at the psychiatric institute.
Payton left hospital seven days after the attack and returned to school in 2014.
