What would you do if you heard a teenager express a desire to kill 500 of his fellow students, go on a shooting rampage in his neighbourhood, and then hijack a plane and crash it in the heart of New York City?

You might laugh and tell the gawky, spotty adolescent to "get a life".

Eric Harris, an 18-year-old pupil at Columbine High School, had no intention of getting a life.

As he shared his violent, nihilistic fantasies with friends, with his diary, with visitors to his Website, he was making preparations to turn his dream of destruction into bloody reality.

So the most pressing question for anyone who visits the poignant flower-strewn shrine in Littleton, Colorado, honouring the 12 students and one teacher murdered by Harris and his accomplice Dylan Klebold, is this: What turned two boys from middle-class, comfortable homes into demonic killers?

Contrasting portraits

There are two sides to the story of the massacre at Columbine High. Talk to friends of Harris and Klebold and you form a picture of two highly intelligent, lonely, alienated teens.

Dark music does not set these teens apart

They hung around with a crowd known as the Trenchcoat Mafia, they listened to dirge-like rock music and surfed the Internet, but there was nothing strikingly different about them.

Their hatred for the school's star athletes, the "jocks", was no secret, but it seemed nothing more than a routine expression of high school tribalism.

Both came from relatively affluent backgrounds - the Klebold family lived in a stunning timber and stone house squeezed between rocky outcrops south of Denver.

Eric Harris's father was a decorated former Air Force pilot.

They ran into trouble with the local police just once, when they were caught stealing from a van. As part of their punishment they had to attend a counselling programme and an "anger management" course.

Both emerged with high praise. Counsellors said Eric and Dylan were capable of doing great things with their lives.

Warning signs

But there were plenty of warning signs for those who chose to heed them.

For more than a year, Eric Harris's behaviour hinted at the violence to come:

• He issued death threats to a former friend at Columbine High.
• His Website referred specifically to home-made pipe bombs he was making and was peppered with violent, Nazi imagery.
• At various times students reported his threatening, sometimes racist behaviour.

No easy answers

Calls for gun control are strong, for now

Now, of course, there are thousands of instant experts prepared to condemn the police and the parents of Harris and Klebold for failing to foresee the violence at Columbine High.

But surely the lesson of the Littleton massacre is that blame cannot be easily apportioned. There is no single, or simple, explanation for the evil unleashed a week ago.

Doubtless the culture in which the two teens immersed themselves played a part. Their obsession with violent video games and movies has been well documented.

But millions of other adolescents share a similar passion and are not transformed into mass murderers.

Conservative thinkers and politicians see the Columbine tragedy as a commentary on the decline of Christian values in America, but during my days in Littleton I was struck time and again by the moral strength exhibited by the Columbine student body.

Using the massacre to condemn a "sick generation" of American youngsters is both unfair and unhelpful to the debate about teen violence.

More interesting has been the reaction of some clinical child psychiatrists who have talked about the physiological origins of some forms of extreme behaviour.

They point to studies that link violence to levels of seratonin in the brain.

Could it be that some children are genetically predisposed to commit acts which society deems to be evil?

The danger is, of course, that you absolve individuals, be they children or adults, of responsibility for their actions. Surely no excuse could, or should, ever be made for the horrors committed by Eric Harris and Dylan Klebold.

Awash in arms

Perhaps the most important element of the post-Columbine debate concerns guns, and efforts to tighten America's gun laws.

While it may never be possible to untangle the web of factors that prompted two angry adolescents to commit an act of mass murder, it is relatively easy to explain how they were able to kill and maim so many.

America has more than 200 million guns on the streets

Simply put, it was easy access to an arsenal of lethal weapons. Bought at gun shows and through friends of friends.

It was the by-product of a society awash with firearms, where possessing a weapon is seen as a constitutional right and gun control is seen as a threat to life and liberty.

Fruitless debate?

President Bill Clinton says he's now committed to imposing new restrictions: raising the age-limit for possession of a handgun to 21 and making parents criminally liable if their weapons fall into their children's hands.

But Congressional Republicans have already made plain their lack of enthusiasm for new gun restrictions and when memories of Columbine High have begun to fade, the president's proposals are likely to wither away.

Anyway it's too late to have much faith in gun control. There are more than 200 million guns circulating through the United States. Any teenager intent on violence can find a way to acquire a weapon.

Apprehensive, tearful students from Columbine High will make their way back to class this week.