Imagine your telephone ringing in the middle of the day, at home or at your work, and being told by a neighbour there is a shooting going on at your child's school.
You immediately flick on the television, or the radio, and your disbelief is dissolved. But there is more.
Bombs are exploding, at least 40 teenagers are trapped inside the school library, the SWAT team has arrived, 300 policemen are on the scene, but no one is exactly sure what is going on. Frantic calls from cellular phones are being placed to police and the media from students trapped inside.
All anyone knows for certain is that a group of students have decided to wage warfare and that they are heavily armed with the means to do so.
You rush to the school but can't get behind the police barricades. Finally you run to the community library beside the school where a command centre has been set up.
"Is my child safe?" you cry out to one of the uniformed policemen. But there is no list of names to check, no confirmed dead or injured. Instead, he can only give you a single directive.
Please go home and bring back dental records or fingerprints, anything that could be used to identify him, you are told.
That was what happened for thousands of parents in Littleton, Colorado, on Tuesday. The town of 35,000, a former ranching community founded in 1890, was enjoying a typical spring day.
It is a typical American suburb, but it is wealthier than most. With a median income of $42,000 in 1994 (25 per cent higher than the national average), Littleton boasts 17 shopping centres and a municipal tree committee to help the city's arborist. They even have a programme to recycle leaves.
But an admonition to retrieve your child's dental records is an abrupt wake-up that your community is not typical.
As police reached the second-floor library at Columbine High School, named for the Colorado state flower, they found 15 bodies, most with multiple gunshot wounds from large-calibre firearms discharged at close range. The two suspects, Eric Harris (18) and Dylan Klebold (17), lay dead from self-inflicted wounds to the head. One wore a bullet-proof vest, and both wore heavy ammunition belts.
But Harris and Klebold wanted the killing to linger on, in fits and starts, even after they themselves were dead. Beside their bodies lay pipe-bombs and guns. In the hallways and corridors, other bombs lay ticking. Dr Chris Colwell, an emergency room doctor at the Denver Health Medical Centre, stepped carefully around the bodies, pronouncing the deaths.
"It is the most difficult scene I've ever encountered," Dr Colwell told the Denver Post.
The police told anxious parents the bodies would not be moved until the campus was secured.
As of late yesterday afternoon, the 15 bodies still lay in the school. Slowly the three area hospitals, where the injured were admitted, began identifying their patients. Those still missing were identified as the dead. Distraught parents of the dead students could only stand by and wait.
Because, as the story of Harris and Klebold began to unfold, police realised that this massacre in a Colorado suburb had a vicious texture and complexity far beyond even the most brutal of the nine school shootings that have plagued the US in the last two years.
Harris had parked his BMW in a different area than usual in the school parking lot. The bomb squad soon found a booby-trapped explosive device. Earlier, a student named Brooks Brown had approached Harris as he was unloading duffle bags from his car.
Harris warned Brooks Brown to leave the campus, that something bad was going to happen.
The scattered bombs soon became evident after the shootings. In a search of Harris's home police found another bomb. Several of the bombs were equipped with sophisticated timing devices. One went off near the school at 11.15 p.m. on Tuesday night. In all, 30 bombs were found, said Jefferson County Sheriff spokesman, Mr Steve Davis.
There were, as there always are in the stories of human beings who commit terrible acts, conflicting accounts about Harris and Klebold. One freshman, a 15-year-old named Jaim, said the Trench Coat Mafia designation of the two and their friends was being overplayed.
"They were just very quiet, they didn't talk. I never saw any swastikas or anything like that. One of them wore a T-shirt that said `AOL - Where the Cool Hackers Are'," she said.
Another student, Nick Baumgarten (17), was good friends with Klebold until a few years ago. In talking about his friend, Nick, a tall, gangly teenager with a thoughtful demeanour, seemed to be both pained and cautious about how the two were being portrayed.
"I went to grade school with Dylan. We were in Boy Scouts together. He and Eric were both really smart kids. They were just really quiet."
Nick said his friendship with Dylan broke up after Eric Harris became prominent. Eric and Dylan would spend hours playing violent video games.
"I don't have anything against video games, even violent ones, but this was really intense. They had computers and this modem thing where they could hook up and play this game called Death Match against each other from their homes. It was a really violent game," said Nick.
When the shooting began Nick was in a class, firing tiles to decorate the school hallways. A group of students streamed past the door, screaming that someone was shooting.
"Our teacher, Mr Manuela, opened the door and stuck his head out. When he brought his head back in . . . I will never forget the look on his face. His face was white. He just told the class to run. By the look on his face, everybody knew it was no joke," said Nick.
The how-could-it-happen-here refrain also began in earnest. Strangely, however, Littleton, Colorado, looks a lot like the kind of place in the US where these things happen. Which is to say that school violence can happen anywhere.
Littleton's population is 93 per cent white. The school, whose motto is "Rebel", is known for its athletic programmes and prowess. It could be a bitter place for those who did not fit in. And those who do not fit in, who are bright and angry, seem to be among the most vulnerable to violence.
A study by the National School Safety Centre shows that school deaths have occurred in all but 11 US states since 1992, a total of 235 violent deaths in school. Those statistics include suicide, and some 14 per cent were gang-related, mostly in urban schools.
But it is the violence of the loners like Harris and Klebold that is fascinating and horrifying to observers. Shunned by peers, teenagers like them often turn to a social world defined by computers, the Internet and video games. According to early accounts, both suspects came from good loving families, a trait they have in common with several other suspects in school shootings.
Why this? Why did they choose this kind of way to show their anger? That was the question on the minds of students and teachers gathered in the school parking lot in the aftermath, milling about, trying to make sense of the senseless.
A teacher at the school, her eyes teary, observed the irony that the two outcast boys would have chosen the library to shoot people.
"They were bright and they read. They loved that library," she said.