It's the question on everyone's mind. How could the parents not know what Eric Harris and Dylan Klebold were up to?
Linda Pollock, a neighbor of Harris' says, "With all fairness to them as far as being a parent and raising teen-agers, you can't know everything all the time."
The fact is, as CBS News Correspondent Richard Schlesinger reports, teen-agers everywhere keep a lot of their lives secret.
The Webne Family in Alexandria, Va. is about as close as any family with teenage children can be. Ben Webne is 18 years old, gets good grades and has been accepted to a top college. Like a lot of his friends, he lives a large part of his life in a world his parents know almost nothing about.
He says his parents are probably aware of about 60 percent of what's going on in his life - which means there's 40 percent that they don't know about. Another high school student, Tracy Tilghamm, says "I know my mom has been through a rough time, and I'm like, my problems aren't as big as hers right now."
What happens in that secret world known only to teenagers?
Sociologist Barbara Schneider of the University of Chicago, explored that world for five years with more than 7,000 teen-agers. "I don't think for the most part most adolescents are in deep trouble," she says.
"Some parents, when kids get to be 16, start feeling they're becoming adults, they're becoming responsible, they're on their own," says Schneider. "And they pull back and they stop parenting in a lot of ways."
In fact, Schneider's research shows most teen-agers rely on their parents for emotional support even while they keep their secrets.
Patrick Egan is Ben Webne's classmate. He says he shares 60 percent of his life with his father, but he's reluctant to talk to his dad about life behind the scenes. So what's in that 40 percent of his life that his father doesn't know?
"Like close personal relationships that kind of stuff," says Egan. "You know, like after school, sometimes after a show, for example, we'll all go out to a party or go hang out somewhere. He doesn't have to worry like that because he knows he trusts me."
Most kids can be trusted. Still, those two in Littleton were apparently able to buy all those guns and build all those bombs right under their parents' noses.
Barbara Webne, Ben's mother, says, "I've thought about that since Littleton. And the fact that they said these children had guns in their bedroom, well, you know I wouldn't know if they had guns in their bedrooms. I just have to assume that from what we know about them and how we raise them they're not going to have guns in their bedrooms."
And even kids who would never get guns or ombs say it could be done.
Egan says, "I don't plan on trying to, but I'm sure that if I really wanted to I could find... I could get my hands on a gun." Without his father knowing? "Yes, because me and him can go for quite a while without actually seeing each other."
Most times, according to experts, teens' secrets are as mundane as everyday life. But there are obvious warning signs that something is wrong -- low grades, aggressive behavior -- and every parent should notice them, if they are paying attention.