The story sounds like an urban legend - but it's real.
On Saturday morning, a bicyclist found a 12-year-old girl in the middle of a path in Waukesha, Wisconsin. Her clothes were caked with blood.
She had been stabbed 19 times - but she was still alive, and had crawled out of the woods for help.
"Who did that to you?" the bicyclist asked the girl during a call he made to 911.
The answer, according to police accounts, would be as shocking as the reason she'd been attacked in the first place - with the Internet being blamed as a "dark and wicked" influence.
Morgan Geyser and Anissa Weier, both 12, have been charged as adults on suspicion of attempting to murder their friend and fellow middle school classmate after inviting her for a sleepover on Friday night.
The plot was months in the making, police said, apparently inspired by a digital-age urban legend named "Slender Man" - a tall, not-quite-human figure with spindly fingers and an empty face who shows up in the background of photos from haunted places.
Slender Man is fictional. But Morgan and Anissa had apparently decided he was real enough that they wanted to please him, investigators said.
After luring their friend into the woods, police said, one of the girls held down the victim while the other stabbed her in the arms, legs and torso.
After her assailants left, the victim was discovered. She was hospitalized in stable condition, police said, but had to undergo surgery for her injuries.
The case prompted Waukesha Police Chief Russell P. Jack to warn the other parents in his community, about 30 kilometres west of Milwaukee.
"The Internet has changed the way we live. It is full of information and wonderful sites that teach and entertain," Jack said in a Monday news conference. "The Internet can also be full of dark and wicked things. ... Unmonitored and unrestricted access to the Internet by children is a growing and alarming problem."
The source of Jack's caution can probably be attributed at least in part to the confession one of the girls allegedly made to police about a website called Creepypasta Wiki.
According to the website, "a creepypasta is a short story posted on the Internet that is designed to unnerve and shock the reader." Creepypasta Wiki is a user-edited database of those stories, similar to Wikipedia.
But the girls were apparently taking the website much more seriously than that. Anissa told police that Slender Man was the "leader" of Creepypasta, and that she and Morgan wanted to become his "proxies" by killing someone - and proving his existence, according to charging documents obtained by local media.
As best as anyone can tell, Slender Man was first created on a general-interest Internet message board as part of a Photoshop contest in 2009.
As with UFOs, haunted houses and Bigfoot, Slender Man has since become a popular staple in the gothic corners of the Internet, which finds itself playing defense.
"I'm going to make myself loud and clear: ALL WORKS PRESENTED ON THIS WIKI AND OTHER SITES (INCLUDING SLENDERMAN ... ) ARE FICTIONAL STORIES AND CHARACTERS," a site administrator identified as Sloshedtrain wrote Tuesday.
"This wiki does not endorse or advocate for the killing, worship, and otherwise replication of rituals of fictional works," the administrator wrote, also offering condolences to the families affected by the attack. "There is a line ... between fiction and reality, and it is up to you to realize where the line is. We are a literature site, not a crazy satanic cult."
On Tuesday, an attorney for Morgan interviewed his client, then told the Milwaukee Journal Sentinel he was seeking a psychiatric evaluation for her.
San Francisco-based psychiatrist Laura Davies, who works with children, adolescents and adults, said sometimes two children can fall into a syndrom called folie a deux, in which two people can share a delusional idea.
"This is clearly psychotic - not necessarily both of them are psychotic, but at least one of them has to be psychotic for this to occur," Davies explained.
From what's been published about the case so far, she added: "This is explicitly not a Internet problem. There's a lot of things I don't like about the Internet, and I'm a big fan of limiting screen time ... but this is an issue of, (one of these girls) is really, really way more than troubled."