The Strand had more than 100 shops, set over two floors. It was probably around the time the boys were seen tapping on the window of a shop on the upper level, trying to beckon a toddler outside, that Denise Bulger was queuing downstairs in film processing store The Photo Expert with her two-year-old son, James. Accompanied by her brother's fiancée and her brother's little girl, Denise took a restless James into shops including clothing retailer Marks & Spencer and supermarket chain Tesco before ducking into A. R. Tyms Butchers. Denise wanted to buy some chops for her husband Ralph's tea.

A. R. Tyms was to be their last stop; after that the group planned on heading back to Kirkby, a tough, unemployment-hit suburb the press called "baby Beirut" in this impoverished pocket of north-west England.

The pair of junior truants had been nicking stuff all morning: batteries, a clockwork soldier, some blue Humbrol model paint. At some point they decided to nick a toddler. It was a terrible dare shared by two damaged children with warped chemistry; it seems fair to say neither boy would have tried to take a child on his own. They managed to coax a youngster a few metres out of department store T. J. Hughes before his anxious mother ran out and scooped him up. Undeterred, they looked around the lower concourse - which still has the same beige tiles and strip lights - and saw little James Bulger standing in the doorway of A. R. Tyms.

Denise only turned her back for a minute. She'd taken out her purse, and the assistant had got the order mixed up. The next thing, James - a Thomas the Tank Engine-loving livewire with a healthy curiosity, a ready laugh and an innocent's trust in human nature - was gone.

"I remember the Tannoy announcing that a little boy was missing," says Gaynor Davis, 44, who was working in The Photo Expert on Friday, February 12, 1993. "That evening the police asked if we'd come back into the Strand so they could check to see he wasn't locked inside somewhere."

Davis answered the phone and grabbed the shop keys: "They knew the little boy and his mother had been in here earlier; they thought he might be curled up in one of the big wooden pigeon holes where we used to keep the prints."

Davis pauses, her gaze steady. A stuffed toy beagle and a tiny watering can, props for the children's studio portraits that are available all day for only £5.99, sit on a high shelf behind her. "They should have got the chair for what they done, those kids. One of them is back inside now, isn't he?" She puts both of her hands on the counter, steadying herself. "Once evil, always evil."

It is now 20 years since blond, blue-eyed James Patrick Bulger was abducted from the Strand by Robert Thompson and Jon Venables. Twenty years since he was forced to walk four kilometres to a desolate stretch of railway line, where he was beaten, pelted with stones and had blue paint splattered in his eye before being bashed to death with an iron bar.

His little half-naked body was discovered on the tracks two days later on Valentine's Day, sliced in half by a speeding train. A group of boys had found the body - at first they thought it was a broken doll - and had rushed screaming down the overgrown embankment to Walton Lane police station, only a couple of hundred metres away.

"I've dealt with many murders but I've never seen the extent of the injuries that were inflicted on someone incapable of defending himself," says Albert Kirby, 67, who was then head of the Merseyside Police Serious Crime Squad. "You couldn't think the person responsible for this was a child."

News of the murder left Britain reeling. How could two 10-year-olds be capable of such an abhorrent deed? What possessed them to torture and kill a baby (in their separate police interviews, both Thompson and Venables would refer to James as "the baby"), in the most brutal way, for no apparent reason? What kind of nation could allow this to happen? "A nation in moral decay," said the then shadow home secretary, Tony Blair.

Britain's tabloids labelled the boys "evil beasts" and "depraved monsters". Front pages and TV news programs from Amsterdam to Sydney featured that now iconic, blurry CCTV image of the trusting James being led from the Strand by the two older boys, his tiny legs hurrying to keep up. Then, as now, there were so many questions. Did the boys understand the seriousness of their actions? Were they capable of understanding?

At that time, children between 10 and 14 could be tried for criminal behaviour in Great Britain only if the prosecution could prove the offender had known that what they were doing was seriously wrong - a rebuttal of the presumption of doli incapax (the incapability of criminal intent).

Bulger's prosecutors did this, paving the way for Thompson and Venables to be tried in an adult court. The oak walls of the old-fashioned Crown Court in Preston, Lancashire, featured gargoyles and paintings of 18th-century judges. Its dock was specially raised so that the two boys could see over it.

"They knew what they were doing was wicked," writes Ralph Bulger in My James, a memoir to be published this week. "Thompson and Venables had plenty of opportunities to walk away from James, to let him live, but they never showed an ounce of compassion or feeling for a tiny boy whose life had barely started."

Albert Kirby agrees, saying, "I've always maintained, even as a Christian, that they summarised the true sense of evilness," he tells me in his calm, measured tones.

Many police at the scene were traumatised by the extent of James's injuries (42 in all). Everyone involved - from the juror who recoiled when asked to hold the iron bar that killed James, to Ralph's brother Jimmy, who had the scarring task of identifying his nephew's remains - was psychologically affected.

"What we were faced with then was something none of us [in the police force] had ever faced before," Kirby says. "The only similar case prior to this one was the Mary Bell case from 1968."

Mary Bell was 11 when she was convicted of the manslaughter of two small boys in Newcastle, in north-east England, and 23 when she was controversially released with a new identity. Just as Bell was deemed "evil" by the tabloids then (and with each fresh airing now), so does Kirby maintain that James's killers were born bad - that they are bad eggs, aberrations.

There were other details. Details withheld from the press and from Denise, who at the time of the trial in November 1993 was pregnant with her and Ralph's second son, Michael, and stayed away.

"The evidence came out in court as to what the sexual injuries were," says Kirby. James's foreskin had been forcibly retracted; a battery may have been forced into his rectum. "But because of the additional anxiety this would have caused, the prosecutor and counsel didn't run it on a murder and sexual gratification charge. They ran it on a murder charge." A pause: "At the end the outcome was the same."

After being found guilty in November 1993, Thompson and Venables became the youngest convicted murderers in Britain for almost 300 years. "An act of unparalleled evil and barbarity," said Mr Justice Morland before sentencing them to be detained at Her Majesty's pleasure for a minimum of eight years, the normal substitute sentence for life imprisonment when the offender is a juvenile.

After agreeing that the two boys should be publicly identified, the judge set an order in place forbidding the disclosure of their whereabouts. Outside the court the crowd surged towards the police vans as Thompson and Venables left for their separate secure children's homes, never to see each other again.

In 1994, the home secretary, Michael Howard, increased the boys' minimum sentence to 15 years following a petition from James Bulger's family with signatures from 278,000 people who believed that the duo should never be released. In 1999, the European Court of Human Rights ruled that putting children on trial in such an environment was unfair, and reinstated the original minimum sentence.

"The issue of the minimum tariff the killers had to serve had been a political hot potato ever since the boys' original conviction," writes Ralph Bulger, "and the goalposts kept being moved."

In his 2010 memoirs, former UK prime minister Tony Blair admits he was wrong to try to take political advantage of the killing, blaming it on years of Tory rule: "I took the easy but ultimately flawed conclusion that our society had broken down. Of course it hadn't as a whole, only in part."

It was under Blair's watch that the Crime and Disorder Act 1998 set the age of criminal responsibility in England and Wales at 10 years old, making it one of the lowest in the Western world.

Pam Hibbert, chair of the National Association for Youth Justice and a former manager at Red Bank secure children's unit in St Helens, Merseyside - where Jon Venables was sent after sentencing, and Mary Bell before him - is campaigning for a change in the British law.

"The number of children aged 10 years old who commit very serious offences is low and always has been," she says. "But the response to the Bulger case absolutely closed down any debate over whether the age of criminal responsibility is too low, which I believe it is."

According to criminologist Professor Gwyneth Boswell of the University of East Anglia, "What you have to look at really is each individual according to intellectual and emotional maturity. I don't think children of 10 have developed sufficiently to know the difference between right and wrong. In Belgium, for example, the age is 18."

Albert Kirby has heard it all before. "In this country we're streets ahead by having an age of 10. And now there is even stronger evidence to suggest that boys of that age have the ability to become involved in some quite serious sexual perversions."

In their separate police interviews, both boys refused to answer questions about the crime's sexual element. Venables became hysterical, hitting his father and weeping uncontrollably. Thompson was defiant, incensed at being called a pervert.

Denise and Ralph's marriage broke down and they divorced in 1995. "Denise was a living reminder of what I had lost," writes Ralph Bulger. "I imagine she felt the same when she saw me. I think if we had stayed together any longer we would have ended up destroying each other."

Both went on to remarry and have children with new partners, and now Ralph Bulger is in a new relationship again. Both have continued to express their dissatisfaction with a system bent on "rehabilitating" their son's killers instead of punishing them, deeming the actions mutually exclusive.

Tabloid stories about the boys enjoying special privileges including PlayStations, holiday camps and football matches while they were locked up underscored their belief that Thompson and Venables had got off scot-free.

"I am still full of hate," said Denise Bulger - now Denise Fergus - more than once. "I will do my best to hunt them down," said Ralph Bulger in 2000 when asked how he felt about the imminent release of Thompson and Venables.

"It was like there were two different worlds," states Ralph's older brother Jimmy in My James. "One for the millions of people like me and Ralph ... and the other world that seemed to be inhabited by the few, a lofty elite who appeared to be out of touch with the rest of us."

In 2001, aged 18, Thompson and Venables were given new identities and released on a "life licence" which imposed strict conditions on what they could and couldn't do.

Thompson has allegedly done okay: there were rumours that he'd fathered a child, that he was living with his gay partner in Australia. (The latter rumour was so persistent that the Howard government was moved to specifically deny it in 2006.)

Venables has proved more problematic. At 17, while still inside the Red Bank secure children's unit, he'd been accused of having sex with a female care worker. For a while, once he was out, he appeared to be functioning. Then he started drinking, taking drugs. In 2008, aged 26, there was a scuffle outside a nightclub and an arrest for a small amount of cocaine. Then, in July 2010, he was returned to prison for two years after pleading guilty to downloading and distributing indecent images of children.

So what went wrong? was 18 too young to release a youth who'd been locked up during his formative years to then try and assume a new life under a new name, in a society where he could never tell anyone what he'd done?

"Venables was dealt with in there with kid gloves," says Kirby. "When he went before the parole board, the evidence was there that he wasn't ready. But no doubt about it, he was released because they didn't want to be seen to be failing.

"In the 12- to 18-month period before [his conviction for child pornography], there were a number of opportunities whereby the authorities could have brought him back in and given him further assistance in making that adjustment to life. But they failed to."

"Somebody didn't ask the right questions, or wasn't properly qualified to do the job, or misinterpreted the signs," says Sharon Girling, a retired police investigator and the Child Protection Advocate at the International Foundation for Online Responsibility. "Why wasn't this picked up? There must be something in the risk assessment somewhere. We're lucky he hasn't gone on and abducted another child."

"The whole episode was a huge embarrassment for the government and the legal system," writes Ralph Bulger. "They had tried to 'cure' a savage killer and had failed. I only hoped that lessons had been learnt to safeguard the protection of all children for the future."

Pam Hibbert feels that the blame lies partly in the fact that Thompson's and Venables' identities were revealed at the time of sentencing:

"In the 2009 Edlington attacks" - which involved the torture and attempted murder of two boys aged 11 and nine by two brothers aged 10 and

12 in Yorkshire - "the judge quite rightly took the view that the boys shouldn't be named because it would mitigate against their chances of rehabilitation."

She mentions the case of Silje Redergard, a five-year-old girl who was beaten and left to freeze to death by two six-year-old boys in Norway in October 1994. It's a case that - out of the glare of publicity - saw the boys remain with their families and back in school within the month.

The echoes of the Bulger case are obvious. But the reaction - of the public, of the murdered child's parents - was startlingly different.

"I remember the mother of Silje saying, 'Something very bad must have happened to those boys to make them do this,' " says Hibbert. "Here in Britain it's either good or evil, which is crazy. Surely, if we believe that adults can be rehabilitated, we have to believe the same for children."

For all his talk of "unparalleled evil", after the trial Justice Morland called for a public debate about parenting and the two convicted boys' family backgrounds.

Venables' mother and father had joint care of their three children, two of whom - Jon's siblings - had learning difficulties. At school Jon gave his teachers cause for concern with behaviour that included rocking and moaning, banging his head on his desk and hanging from a coat peg like a bat.

Robert Thompson was the fifth of seven children in a family that was textbook dysfunctional. Abandoned by their father, with an alcoholic mother who had a history of violence and stints in care, the Thompson siblings were streetwise beyond their years. Or seemed to be; Robert was bullied by his brothers for sucking his thumb.

Thompson used to pick on Venables until they were kept down a year at school and put into the same class. They forged a common bond by bunking off school, shoplifting and mucking about; they liked frightening elderly ladies by jumping out at them in the street.

Each boy blamed the other for the murder. The 38 witnesses who saw the boys walking the route with James didn't single out either of the boys as the main culprit. Thompson was held to be a sort of malevolent Pied Piper, leading the impressionable Venables on a macabre dance.

When the news came in 2010 that one of James's killers had been recalled to prison, most people assumed that it was Thompson.

"When I first met Jon I would never have believed he was involved," says Laurence Lee, Venables' then solicitor, when we meet in a cafe opposite the Crown Court building in central Liverpool. "He looked like an angelic eight-year-old, and was a very convincing little liar. At first I thought, 'What am I doing here? He's never been near the Strand!' Until Thompson admitted to being there in his interview and that was put to Venables, and the screaming and wailing began: 'Okay, I might have been there, but I never killed a kid!' "

Lee, 59, blinks behind his wire-rimmed specs. "That was a spooky, scary moment," he says. "I think Venables realised straight off - as much as a 10-year-old can - that his life was over.

"He was a much more vulnerable child than Thompson. Thompson was hard-headed enough to get on with life; Venables, unfortunately, hit drink and drugs very quickly. He might have been at liberty, but he was never free."

It wasn't until the case was over that Lee started having nightmares about being run over by a ghost train: "You didn't have post-traumatic stress in those days. Today I'd probably be off on six months' paid leave."

It's a bright winter's day in Liverpool. The city has blossomed in recent years, and wherever I go, I'm met with the same open friendliness. Everyone remembers the Bulger case. Even if they were just children themselves, even if they weren't even born then, they know. Because in a way it wasn't just Thompson and Venables on trial, and their parents on trial-by-media with them (both families moved away after their sons' convictions and took on new identities). It was as if the city of Liverpool was in the dock as well.

"Liverpool was a very different city then to what it is now," says Mark Thomas, editor of The Liverpool Post, and author of Every Mother's nightmare: The Murder of James Bulger. "Liverpool had come through 15 years of real hard-core economic depression and was a pretty demoralised region at the time," he tells me as we sit in his office overlooking the busy newsroom. "But the one thing the Hillsborough disaster helped generate was a spirit of unity, a sense of protectiveness and closing ranks."

Four years before James's murder, 96 Liverpool football fans had been crushed to death at Hillsborough stadium in Sheffield. The Sun newspaper erroneously pinned the blame on the Liverpool fans, on a bunch of drunken Scousers misbehaving at a football match. The lie cut deep - but it brought Liverpudlians together.

Some 23 years later, on September 12, 2012, following the Hillsborough Independent Panel's report, The Sun (still commonly referred to in Liverpool as "The Scum") finally issued a formal apology to the people of Liverpool.

"But this [the Bulger case] concerned two people from within Liverpool's own community, committing an unimaginable crime on another child from within that community," Thomas says. "People here didn't know how to rationalise that."

It was easier, perhaps, to insist that the boys "were born evil", to label them as psychopaths. This vindicated the mobs who pounded

their fists on police vans, who were sure that Thompson and Venables would kill again, who advocated locking them up and throwing away the key. In that sense, it absolved them of any collective responsibility.

But like Hillsborough, it also brought people together. There is no doubt that the mass weeping for James, that the laying of flowers, soft toys and handwritten cards on that stretch of Walton railway line, was borne of genuine feeling.

"Self-Pity City" ran a headline in The Sunday Times, for which it, too, later apologised. Such accusations of mawkishness only served to strengthen the divides thrown up by the case: working class versus the chattering classes; rehabilitation versus punishment; good versus evil.

For a long while, Denise Fergus was treated with something approaching disdain by the liberal media. The marches, rallies and publicity that she organised through her own justice campaign variously saw her riding in the cab of a truck draped in "Justice for James" banners on the day of Thompson and Venables's release; demanding to see photos of the adult Thompson and Venables (arguably leading to vigilantes posting photos on the internet); and in February 2012 speaking out against Robert Thompson's plans to sue News International, publishers of the now defunct News of the World, after police confirmed that his phone had been hacked. "It would be a terrible insult to James's memory if the animal who murdered him was to receive a payment," she told the Daily Mirror.

Fergus didn't give articulate interviews speaking about forgiveness for her son's killers, like Silje Redergard's mother. She wasn't awarded an OBE for her services to community relations like Doreen Lawrence, mother of the murdered black British teenager Stephen Lawrence. Indeed, by demanding to know why the adult Venables was recalled to prison, she was held to be stirring up a lynch mob. She was too angry, too raw, too working class. "Tormented as she undoubtedly still is, she clearly revels in a role for which her background makes her perfectly suited," wrote Charlotte Raven in The Guardian in 2001.

Arguably, Fergus wasn't the right kind of victim. "The really sad thing about these two [Ralph Bulger and Denise Fergus] is that no one seems to have helped them move forward," says criminologist Gwyneth Boswell. "It seems to me they've been encouraged to continue the hatred. You've only got to look at their faces to see this hasn't helped them."

"You can't tell people how to channel hatred, or how to mourn," says Laurence Lee. "People grieve in different ways. Denise has been tremendous in her charitable work, especially of late." Fergus currently helms the James Bulger Memorial Trust, a charity founded on March 16, 2011 to mark James's 21st birthday, which offers support to young people who have become the victims of crime, hatred or bullying. "James lives on as far as she is concerned," adds Lee. "Quite rightly."

In June 2011, Venables lost his bid to be released on parole for a second time. In November of that year, it was reported that he is being kept in prison for his own safety - because he can't be trusted to keep his identity a secret. "He used to say to me, 'I don't want to get out, 'cause I know what will happen to me,' " says Lee. "From an early age he was looking over his shoulder. He was always scared.

"This case was so unique at the time, children killing children. Youth crime is a lot worse these days ... Ten is the right age of criminal responsibility. I don't care what the do-gooders say."

Lee stands to leave; he's due back in court to help prosecute a drug dealer, a local Mr Big.

"Have you been down to the Strand?" he says. "It hasn't changed one iota. It's a time warp."

Except that some of the shops that were there on that February day in 1993 aren't there any more. T. J. Hughes department store is an empty shell.

"My son was the same age back then," says Jenny Johnson, 45, at Discount Fruits. "We all went out and bought them child reins; suddenly there were all these little kids on leads everywhere."

"[James] was naturally nosy and friendly," writes Ralph Bulger, "and he would have been in seventh heaven looking around at all the shops and all the people there that day."

What was A. R. Tyms Butchers is a charity shop selling second-hand furniture and homewares.

"There's no one in Bootle who doesn't know the Bulger case," says a strapping young sales assistant, John Sefton, who tells me that he lives out by Kirkdale Cemetery, where James Bulger's grave is tucked away under a tree with a sign that reads "James's Special Place". "I'm the age that he would be now," he adds.

I'm taken aback. With James Bulger frozen in time as a smiling blond toddler, it's easy to forget he'd also be a young man today had he lived.

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