>> reporter: geyser and weier plotted for months for the best way to kill her friend. duct tape her mouth and stab her in the neck and then changed their plans to lure her into the woods while playing a game of hide and seek. the motive, say police, to win the faye very of a fictional internet horror character. the girls found on the horrow fantasy website creepypasta.wiki.com. >> it was a fictional character created as part of a joke in 2009. most of his representations slenderman is a thin, tall, faceless character. the creepy part about the character is in most stories he kills children. >> slenderman character, as i started looking into it, he's everywhe everywhere. >> yes. >> is there a sense they were reading about him and studying him in various forms online, not just creepypasta? >> as far as all their internet sites, we're going the computer forensic evidence of computers and such. i would assume it will turn out