Sometimes it must seem to the public that the more researchers learn about homicidal teenagers, the less we really know.
For example, we suspect many people understand that quite a few violent rampages by teenagers and pre-teens involve "loners" - youngsters with few or no friends, who just don't "fit in."
But police in Murrysville, Pa., say that was not the case with Alex Hribal, the 16-year-old who went on a slashing, stabbing spree last week at his high school. Twenty-two people were hurt.
Hribal's attorney, Patrick Thomassey, pointed out that in many school violence tragedies, the perpetrators have been teens who "had problems in the past or [were] considered by other students to be an outcast." But other students told him Hribal "wasn't a weirdo. He was, for the lack of a better word, a fairly popular kid. He just was one of the guys. He fit in well."
Hribal had no history of mental illness. He didn't use illegal drugs. He apparently had a good family life.
On down the list of signs of troubled, potentially violent youngsters we go - but none of the boxes are checked for Hribal.
Obviously, something prompted him to take two knives to school, then begin using them on fellow students. Hopefully, researchers will be able to discover what triggered Hribal.
But it is his apparent normalcy that is most unsettling. Can we have no faith in what studies tell us about violent teens?
Yes, we can place some reliance on researchers' work. Among the first things most tell us is that efforts to determine what makes some people violent are in their infancy. We know a few things - that children who hurt animals are more likely than others to harm humans, for example. But there is so much we do not know.
Hribal is not a reason to mistrust what we do know. Instead, he is more bloody evidence of the need to learn more.