John DeStefano stands apart on a snowy rise near Columbine High School. The grass has been trampled as people keep coming in an extraordinary outpouring of grief. Students break into song, form prayer circles and tell their stories to newsmen and cameras. The rise is becoming a shrine as people bring flowers. There are flowers too in the nearby parking lot, festooning a car of one of the dead students. The snow, which fell heavily after the massacre in this small Colorado town on the outskirts of Denver, is beginning to melt. “We couldn’t have prevented this rampage,” says DeStefano. “We can’t even prevent the assassination of a President.” The president of the Jefferson County School Board breaks down and is unable to speak.

Two days later the school’s principal Frank DeAngelis is struggling for words at his first interview. He tells how he saw one of the teenage killers heading towards him down the hall, shooting into windows as he came. “I thought it was all over for me. I ended up going down a hallway. I’m not sure why. I guess it wasn’t my time...he didn’t come after me.”

He tells too of a new, deep anxiety. How can he reassure parents that their children will be protected in future? “Until Tuesday I could stand up and tell them that I would be responsible for providing a safe environment for their sons and daughters.”

John DeStefano is sure of one thing. “If it could happen here, it could happen anywhere.” “Here” is a fast-growing community of 35,000 people in new suburban estates on the fringes of Denver. It’s Dan Quayle country: white, comfortable, Christian - and it used to be safe.

What can American teachers do when faced with teenagers like Dylan Klebold, 17, and Eric Harris, 18, members of the Nazi-worshiping Trench Coat Mafia, who ran amok in the Littleton school on Hitler’s birthday before they killed themselves? The labels fly: early intervention, anger control, peer mediation, and schools where everyone from social workers to nurses, janitors and bus drivers are taught to look for danger signs. Oklahoma, for example, now has a 24-hour hotline for parents, teachers or children who feel threatened at school. Uniforms have been brought back, badges with hate messages banned, metal detectors installed and random searches started.

Some experts say the answer lies in Secret Service “threat assessment” techniques. After the shootings there were calls for teachers to be trained in how to spot depression and violent tendencies - and to act on their perceptions.

“Schools do vision and hearing screenings,” says Dr Harold Koplewicz, of the Child Study Centre at New York University. “Doesn’t it make sense that kindergarten teachers learn how to screen for aggressive behaviour?” Early this year the Jefferson County school district, which includes Columbine, asked every one of the 140 schools and education centres in the district to adopt a “violence response plan”. Just one week before the killings, staff at the high school held a training meeting, practising their plan and teachers’ responses. Now they are credited with saving many lives - staff knew to raise the alarm, to stay calm, to make the right call of when to run and when to stay put.

One early lesson from Columbine is that crisis training counts - and that staff reactions are critical. An armed police officer had been assigned to the high school - as the gun lobby has recommended for every school - and reportedly exchanged shots with the assailants but could not stop the massacre. It took four hours before America’s much-vaunted police SWAT teams discovered the bodies of the gunmen.

What do America’s school shooters have in common? “Guns, and anger,” says one teacher. “Bright and troubled,” says another. They are often described as the boys that no one notices; difficult relationships with parents crop up - in some cases they are the first victims.

Some of the problems seem unique to the era - intensely violent films and video games, the electronic literature of the American far right. In Columbine there was a sense that victims had become like targets in a video game.

Often the killers wear trench coats or camouflage pants. Luke Woodham, who shot two classmates dead and wounded seven others in October 1997 at Pearl High, Mississippi, was a member of a cult-like group, “The Kroth”. In Paducah, Kentucky, 14-year-old Michael Carneal wrote a secret story about a shy boy named Michael who was picked on by “preps”, popular classmates, and was saved by a brother with a gun. Carneal gunned down eight classmates at a prayer circle in Heath High, Kentucky, in December 1997.

Woodham killed his mother, with a baseball bat. Kip Kinkel, in Springfield, Oregon, last year allegedly killed both his parents, teachers in their 50s.

Kinkel had an overwhelming obsession with guns and explosives, wrote violent threats in his journal, and was voted “Most Likely to Start World War III” by his classmates. He was also in an anger management programme, and had made a speech in class about explosives.

Dylan Klebold and Eric Harris left their videos, their hate poems, a web site so violent and angry that it had been monitored by national watchdog groups. The pair lived with their middle-class families in lily-white neighbourhoods, in a school where their privileged past-times included working on the broadcasting unit and a class in film-making. One video they made showed teenagers stalking corridors, shooting at classmates who played the roles of “jocks”, sporty types, who were said to be singled out as targets in last week’s massacre. Klebold, at elementary school, was in a programme for high-achievers.

Columbine teacher Cheryl Lucas says administrators ignored her warnings about the pair and their potential for violence. “We did see signs that these kids were disturbed.” Teachers knew about the Web pages and “hate stories and hate letters,” she says. “But as long as their threats are general, nothing much can happen. There’s nothing that permits us to take action based on signs.”

Teachers in these crises are often the most traumatised - and the last to speak out. They are protected by school boards’ public relations staff and tread carefully through a litigious American culture. At Columbine, three days into the drama, Frank DeAngelis and his 110-strong staff were nowhere to be seen. The story was left to their 2,000 students who told of a succession of teachers herding them out of harm’s way, into classrooms and under cover. “She was shaking, but she maintained a strong voice,” says Nikki Simpson, 15, of her teacher Cheryl Mosier. “She locked the doors of our classroom for us.” Mosier handed out pencils and paper; some children played noughts and crosses, others wrote letters to parents.

“When the alarm went on, the class thought there was a fire, but the teachers came in and told us to stay down, stay still, and not talk,” says Jeff Williamson, 15. They took cover behind up-ended tables. “The teachers were always there, they didn’t run and leave us.”

David Sanders, the teacher who raised the alarm then saw other pupils to safety, was shot twice. He died in the arms of students who took instructions from medics on their cellphones to try to keep him alive.