On an April morning in 1999, two teenagers, Eric Harris and Dylan Klebold, walked into Columbine High School in Colorado and murdered 12 fellow students and a teacher, wounded many others, then turned their guns on themselves. Among the many questions fired at Klebold’s stunned parents in the wake of this appalling event, two were especially hard for them to hear. Did they ever hug their kids? And — this from one of the many bereaved — ‘Were you a family that ever spent much time at the dinner table together?’

When brutal and frightening things happen, people want brutal and frightening explanations: the need for causality becomes paramount. If this family bred a monster, then they must have done something monstrous. For if we can’t reassure ourselves that such a thing won’t — can’t — happen to us, then surely no one is safe. Much to her credit, this is the still-urgent question that Dylan’s mother, Sue Klebold — now a suicide-prevention activist — dares to explore some 16 years on, in her agonisingly self-scrutinising and morally acute memoir.

The uneasy truth is that the Klebolds seem to have been a middle-class family just like so many others. Happy, loving, socially and emotionally aware, they not only did very much eat dinner together, but also, like the rest of us, did their best to limit the ‘intake of television and sugary cereals’ and monitor ‘what movies our boys could see and put them to bed with stories and prayers and hugs’. And of the two children, Dylan was for a long time ‘the classic good kid… easy to raise, a pleasure to be with, a child who had always made us proud’.

But it turns out that this ‘easy’ young man was also deeply troubled — a ‘perfectionist’ whose ‘unrealistic expectations for himself’ seem to have resulted in (never diagnosed) depression, contributing to overwhelming ‘alienation at the end of his life’. There is no doubt that in the months before he died, Dylan was experiencing frequent suicidal thoughts. He did not, it was later asserted, have the ‘profile of a killer’ but (crucially, as we now know) he had the ‘vulnerability to become enmeshed with one’. In fact a fascinating difference between the two boys identified by an FBI psychologist was that ‘Eric went to the school to kill people and didn’t care if he died, while Dylan wanted to die and didn’t care if others died as well.’

But the Klebolds didn’t see any of this. What they saw was exactly what most parents of male teenagers see: a young man who could be moodily and randomly difficult one day and sweet, kind, funny and reassuringly communicative the next. Meanwhile, though, Dylan was recording considerably darker feelings in his private journals.

And in a way, this is the big, slippery conundrum at the heart of the book. If the Klebolds had done what, let’s face it, few parents would do, namely read their son’s private journals, might we never have heard of Columbine High School? Klebold insists that, given her time again, she would do so, chiding herself again and again for not doing ‘whatever it would have taken’ to understand Dylan’s ‘internal life’. But how could she have known what it would have taken — or indeed what the price might be? ‘Dylan was withdrawing from us that year,’ she notes. But what teenager doesn’t? Conceding that she and his father ‘didn’t identify Dylan’s desire for privacy as a red flag’, I think she’s being too hard on herself. What parent of teenagers ever does?

The huge tragedy for the Klebolds was that they didn’t just lose their son on that terrible April day, but also ‘his very identity — and ours’. Their future was changed forever but so, irrevocably and cruelly, was their past. And the aftershocks were many. Months later, sitting in the sheriff’s office, the time came to hear a moment-by moment-breakdown of the violence inflicted by their child. So chilling and traumatising was the account that, forced to realise they had been in a kind of denial, both parents had to begin the grieving process all over again. The boy they had been mourning ‘was gone’, replaced by someone they didn’t even recognise.

There was, at the time, much heated discussion about whether the so-called ‘basement tapes’ — aggressive videos recorded by the two boys before the killings — should be made public. Ultimately it was decided that the risk of their being fetishised by the press, not to mention their potential for inspiring copycat crimes, was too great. But it’s hard not to agree with Klebold when she suggests that ‘it would be far more instructive — and frightening — to show the video we took on the afternoon of his prom, three days before the massacre, smiling and playfully tossing tiny snowballs at his dad behind the camera’.

None of us will ever know how it feels to be Susan Klebold; her particular tragedy is, thank God, precise and rare. But the real triumph of this powerful book is that, in seeking to ‘own the viciousness’ of her son’s ‘final moments on earth’, she has managed to craft something that feels both universal and urgent and which, in its deep seriousness, also honours the Columbine victims. Not least, it’s a very moving memorial to a troubled young man who, both then and now, was clearly very much loved.