For the latest coverage of the tragedy in Littleton, Colo., see our coverage in The Boston Globe's Boston.com. Wednesday's coverage -Rampage reconstruction -Families mourn victims -Colo. gun law debate -What went wrong? -False 'warnings' on AOL -Clinton speaks on massacre -World reactions Globe coverage -Need for prevention -TV coverage -'A suicide mission' Background -Map of Littleton -List of injured -Shooting chronology -Past school shootings -What was 'Trenchcoat Mafia'

The tragedy unfolded in images that have become eerily familiar, panicked groups running for cover, sobbing teens telling how they dodged the bullets, distraught parents clutching at each other for comfort.

Underneath, a question as chilling as the scenes of televised misery: Why are America's schools turning into killing fields?

''Anybody who has children, anybody who doesn't have children should be frightened about what this bodes in a society where we have so many things and so much, and yet we can create monsters that can go in and shoot 25 peers,'' said Dr. Gery LeGagnoux, a UCLA teacher and psychologist.

In October 1997, when a 16-year-old boy in Pearl, Miss., was accused of killing his mother and shooting nine students, two fatally, at his high school, the killings were a stunning anomaly.

By Tuesday, when two trench-coated young men swept through their suburban Denver high school, the phenomenon of students killing students seemed far less abnormal.

Springfield, Ore., Jonesboro, Ark., West Paducah, Ky. - five mass shootings at schools in just the past two years.

Experts don't have one answer for what is behind the shootings; they have a lot of them. Lack of supervision, accessible guns, permissive or absent parents, school officials who fail to act on warning signs, a culture redolent with violence - all are cited as contributing factors.

''These kids have never learned how to solve problems. They have an instant answer, and that's a gun,'' says Bill Reisman, a criminologist who has advised officials from several school districts that were the sites of earlier shootings. Increasingly, the shootings are suicide missions, as was the apparent case Tuesday, he said.

''Most of them believe that death is now the solution and in order to get the maximum amount of attention they do these bizarre, heinous crime,'' he said.

Violent movies and music can add ''fuel to a fire,'' but Reisman argues ''we can't blame things. People are responsible.''

Underpinning those problems is a society in which family and community ties are unraveling.

''It used to be when I was a kid, if I did something wrong down the street, before I got home that neighbor would have called my parents,'' Reisman said. ''These days they're afraid they're going to get sued.''

Peter Blauvelt, president of the National Alliance for Safe Schools, a consulting firm, said it's nearly impossible to find immediate answers to a tragedy like the Denver shootings.

''I think one of the things we are suffering from is trying to rationalize what was really an irrational act,'' Blauvelt said. ''This is a huge tragedy with a tremendous amount of ramifications in the United States, and the whole world for that matter.''

But Blauvelt said there are steps that can be taken, such as keeping a closer eye on class misbehavior - and listening to students.

In the latest shootings, students said the gunmen apparently belonged to a clique of outcasts who wore long black trench coats, boasted of owning guns and disliked blacks, Hispanics and athletes.

''My guess is that there was conversation between and among kids about what was going to happen. And no one listened to it,'' Blauvelt said.''

Reisman had the same advice for school officials: pay attention.

''You're going to have a lot more,'' Reisman said. ''I was involved in Pearl. Every time, this has escalated. The kids have learned from the previous one.''