Denise Fergus still cannot bring herself to walk near the Walton railway line. The track is a constant reminder, a prosaic memorial of all that haunts her. She goes out of her way to avoid it, to circumvent this unremarkable part of Liverpool, even if it adds miles to her journey and makes her late home.

It has been 15 years since the murder of her son James Bulger on this stretch of track; 15 years since he was beaten to death by two killers who were themselves children. Time might have passed but, if anything, Denise's memories have come more sharply into focus with each quiet anniversary. There is nothing exceptional in her annual remembrance, nothing that would intimate the barbaric nature of her son's death nor that would hint to a passing onlooker at the anger that burns deep inside. On the day James was killed, 12 February, she took a wreath of flowers to his grave. The rest she kept in her thoughts.

'It was a difficult day,' she says. 'Getting through February is always hard. James would have been 18 this year. The real sadness is that he would have been so loved.' Instead, his death at the age of two became a part of legal history. When found guilty of the killing in 1993, Jon Venables and Robert Thompson were the youngest convicted murderers in Britain for almost three centuries.

Most of us can remember the Bulger case. We remember the toddler's disappearance - that blurry-edged CCTV footage of James being led out of a shopping centre by two older boys, his hands trustingly outstretched, his small legs whirring to keep up. We recall the sudden horror of his death, the discovery of his body 48 hours later on Valentine's Day and the shocking realisation that the prime suspects were only 10. We remember the mounting sense of horror that children could be capable of such cruelty, later confirmed by the trial judge's statement that theirs was an act of 'unparalleled evil'. We remember that James was just two years old, too young, far too young, to have been dragged under by life's dark undertow.

But, 15 years on, some of the detail is likely to elude us. The macabre precision of the post-mortem examination, for instance, that showed James had been beaten, kicked and bruised by his tormentors, that he had been thrashed with an iron bar and pelted with stones. That he had been forced to walk more than two miles, bloodied and crying, to a desolate stretch of railway line. That his face had been splattered with blue paint and the hood of his anorak had been ripped off. That when he was dead, the two boys laid him across the tracks and buried his head under a mound of bricks. That they stripped him of his trousers, shoes and socks. That a train ran James over with such force his legs were sliced from his torso and flung several metres from his upper body.

For several years after reading the police reports, Venables's solicitor, Laurence Lee, suffered from nightmares. He dreamt that he was eight years old, taking a ride on a ghost train at a local fairground. 'I would fall out on to the tracks, be run over and killed,' he says. 'I'm 54 now but there's not a day that passes without me thinking about it.

'The police found a leaf stuck to the bottom of his bare foot. That one detail broke my heart.' The murder was, he acknowledges, senseless in the truest definition of the word. There was no answer to the perpetual why. There were just more and more questions, heaped upon each other until there seemed little point in asking them any more.

Certainly, the people of Liverpool do not ask. It is too painful to be reminded and too hopeless to seek explanation. It is a place used to tragedy - four years before James's murder, 96 Liverpool football fans were crushed to death at Hillsborough - but one that has also sought to rejuvenate itself. This year, it is the European Capital of Culture and people are wary of bringing up the past, of associating themselves endlessly with tainted sadness.

Yet wherever you go in this city, whichever street you walk down or door you knock, the memories still float to the surface like unbidden driftwood. Four miles north of the centre, in the Strand shopping centre in Bootle where James got fatally separated from his mother as she stood at the butcher's counter, the shadow of what happened still casts its pall over the tinny piped music and cellophane-wrapped teddy bears.

'It's not forgotten by any means,' says Donna Martin, 36, the assistant manager of Clinton Cards. 'I just remember the sheer horror of it. At first, we hoped the little boy would be found and then, when we heard it was kids who had done it... it was just unbelievable.'

Two miles east in Walton, where Venables and Thompson grew up, there is a pervading weariness, a sense that the community has become calcified in a state of perpetual penance. At St Luke's Church, the gates have been padlocked and sprayed with anti-vandalism paint. Last week, the owner of the local newsagent was forced to divide the shop in two with a floor-to-ceiling glass screen after a customer tried to jump the counter.

John, 26, was in Thompson's year at St Mary's primary school. 'I can't really remember him,' he says. 'It's not something you want to remember, to be honest, even though it's always there. Everyone here has been affected by it. A mate of mine found the body. He didn't recognise it as a boy at first. He thought it was a doll. It just destroyed him.'

But it is in a small pocket of the Liverpool suburb of Kirkby that James's death is most keenly felt. Denise Fergus still lives here, in a neat, semi-detached house filled with mementoes: a lock of James's hair, a selection of small jumpers, his go-kart. In the loft, she keeps a fireplace from her old home, marked by a toddler's accidental, greasy handprint. Divorced from James's father, she has remarried and insists on talking about her son every day to her three other children - Michael, 14, Thomas, nine, and Leon, eight.

'I cherish the memories of him,' she says. 'Like a day I'd come out of the shower and sprayed on some deodorant. He said "You smell lovely Mummy", and put a big smile on my face. I still wear that same deodorant today.

'The pain of losing him will never go away. But there's so much more in my life that I determined long ago not to be a victim any more. I don't let things hurt me so easily as I once did. Like it was hurtful when the papers called him "Jamie". That was never his name. It was like a strange label they invented to sum him up in one word. It's the same now with Madeleine McCann. The papers call her "Maddy".

'The thing that rubs salt in the wound for me is knowing the two who killed him are walking around thinking they got away with murder.

'I can never forgive Thompson and Venables for the horrendous, calculated, cold-blooded murder of James.

'They were 10 years of age but much, much older in their minds. They knew full well what they were doing, yet they've never shown a single shred of remorse.'

At the time, none of us was sure what to make of those two young boys, the static grins of their school photographs imprinted so forcefully on our consciousness. In the aftermath of the trial in November 1993, the Daily Star carried pictures of Venables and Thompson underneath the headline 'How do you feel now you little bastards?' alongside the unconsciously ironic masthead slogan, 'The newspaper that cares'. It seemed to sum up society's own discomfort: the conflicted paradox between feeling sympathy for children caught up in something they did not necessarily understand and the primal rage provoked by the murder of a toddler entirely unequipped to defend himself.

It seemed easier to say that Thompson and Venables were 'born evil', to absolve us of collective responsibility, to paint them as examples of a monstrous otherness whose actions were beyond rational explanation. But for some, such as the consultant child and adolescent psychiatrist Dr Eileen Vizard, who gave evidence at the trial, the reasons children kill are more subtle: 'More often than not, it is a complicated matrix of individual, familial and environmental risk factors that come together in a bad way at the same time.'

Both Venables and Thompson grew up in an area where unemployment was twice the national average and both their fathers were out of work. They came from two broken homes and forged a common bond through playing truant. The domestic chaos was worse for Thompson - the family was well-known to police and disliked by locals, and his older brother Ian had taken an overdose of paracetamol to force social services to put him into care.

In the run-up to the Bulger murder, much of what they did was the normal misbehaviour of a pair of youthful miscreants: they would while away their days with bouts of petty shoplifting and frighten elderly women by jumping out at them in the street. But Thompson was, says Laurence Lee, 'like the Pied Piper'. 'When they took James to the railway track, Jon said to me that he only threw small stones, that he missed on purpose. Robert Thompson allegedly said "Are you blind, divvy?" Thompson was the most frightening child I've ever seen, with these cold, steely eyes. It was absolutely chilling when he stared at you.'

Neither boy expressed remorse, although Venables was the more emotional of the two, crying and shrieking as the sentence was passed while Thompson sat, dry-eyed, occasionally smirking, fiddling with a gold ring on one hand. The strength of feeling against Thompson ran so high that a frustrated member of his own legal team is said once to have pushed him up against a wall and shouted 'Why don't you ever cry, you little bastard?'

Did they know right from wrong? Teachers who gave evidence at the trial concluded that they did, but the legal process must have seemed at times beyond their comprehension. At 10, Britain has one of the lowest ages of criminal responsibility in Europe and in Preston Crown Court, the dock had to be raised by 18 inches so that the boys would be able to see over the top. Venables would spend hours playing Tetris with his junior counsel as his legal team tried to extract information. 'He would say "You're not going to ask me any hard questions are you?",' recalls Lee.

In 1999, the European Court of Human Rights ruled that the boys' had been too young to understand the proceedings and their sentence was reduced from 10 to eight years. Released in 2001, Venables and Thompson are living under assumed identities. In detention, they received a better education than many of their contemporaries, each gaining a clutch of GCSEs and A-levels - something that has caused enormous bitterness for those left behind.

Denise finds it especially galling. 'They have been rewarded with the best of everything: a fine education, a new life and protection by the state,' she says. 'They did get away with murder. They got away scot-free. But I am still under a kind of life sentence.'

There is nothing, now, to mark the spot where her son's short life ended with such brutal force. The railway line, approached by a steep grassy embankment strewn with the broken necks of vodka bottles, has been screened off with high metal fences. On one side stands a derelict pub, its boarded-up windows overlooking the track with a blank, shuttered gaze. It is a desolate place to end a life, a bleak memorial. But if there is no physical monument, no plaque or flowered wreath, there is something altogether more lasting. In Liverpool, remembrance is carried close to the heart. It is a city of memories and James Bulger resides there still.

Years of suffering

· 12 February 1993
James Bulger, aged two, goes missing from the Strand shopping centre in Bootle, Liverpool.

· 14 February 1993
Bulger is found dead on a railway track several miles from the shopping centre.

· 18 February 1993
Jon Venables, 10, and Robert Thompson, 10, are arrested in connection with the murder of the toddler.

· 1 March 1993
Thomson and Venables make their first appearance at South Sefton magistrates' court in Bootle and are charged with murder. There are violent scenes outside the court.

· 24 November 1993
Thompson and Venables are tried as adults and sentenced to no less than eight years at Preston Court.

· Summer 1994
Michael Howard, Home Secretary, increases the sentence to 15 years.

· 12 June 1997
Jack Straw reconsiders the sentence for Thompson and Venables.

· 26 October 2000
Lord Woolf reinstates Thompson and Venables's eight-year sentence
after pressure from the European Court of Human Rights.

· 8 January 2001
Thompson and Venables win anonymity for life.

· 22 January 2001
Thompson and Venables are released on life licences by the parole board and are given new identities .