WAUKESHA, Wis. — At Horning Middle School last week, some girls painted cat whiskers on their faces with liquid eyeliner, in tribute to a fellow sixth grader who loved cats but who lay in a hospital recovering from 19 stab wounds that the authorities say were inflicted by two friends who had lured her into a park to kill her.

Overnight, this trim, upper-middle-class Milwaukee suburb, where the school plaque promises that “success is a tradition,” has been forced into urgent self-examination about unfathomable violence — brought on, perhaps, by an obsession with a macabre Internet figure. The police chief, Russell Jack, called the attack on the girl, 12, a “wake-up call for parents” and urged them to closely monitor their children’s use of the Internet, which he said can be “full of dark and wicked things.”

Among these things is Slender Man, a sinister-looking fictional character the two other girls, also 12, cited as the impetus for the attack. They told the authorities that they believed Slender Man was real, that he lived in a mansion in the Northwoods of Wisconsin and that they needed to kill to prove themselves worthy to him. They were charged as adults with attempted murder.

School leaders here are reeling, trying to tamp down rumors about the attack and those involved while maintaining some level of normality for nervous students. Parents are stunned, some considering new limits on play dates, sleepovers, even church youth group gatherings. Students, who were already wrestling with the painful social structure that is middle school, seem perplexed and left with a new wariness about friendship.