They have called it scruffy, cheap and unloved. They have sneered at the wire mesh fences and unmowed lawns and the rusting trucks in almost every driveway. And in time, when the media writes the final chapters of the appalling story of Jaycee Lee Dugard, they may very well conclude that in Antioch, her story was simply an accident waiting to happen.

A staggering 122 registered sex offenders live here, in a small, blue-collar city in northern California that has suddenly found itself at the centre of an international media storm. More than 100 of them – 102, to be precise – live in the compact zip-code area containing the suburb that Jaycee Lee's alleged kidnapper, Philip Craig Garrido, called home.

Two convicted rapists reside on Vine Lane, the street next to Walnut Avenue where Jaycee Lee's imprisonment and sexual abuse went unnoticed for almost two decades. On Viera Avenue, less than 200 yards away, is the home of Henry Lee Mickens, a 46-year-old man who recently served time for "lewd or lascivious acts with a child under 14 years old".

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Dozens of other paedophiles can be found within walking distance. A mile and a half from Garrido's front door is Gragnelli Avenue, where the occupant of No 420, one Shayne Patrick Gaxiola, was convicted of molesting a 12-year-old girl and impregnating her in 1994, when he was aged 20.

Gaxiola was also found guilty of giving marijuana to a string of pubescent girls. He then took indecent pictures of them. In 2000, three months after his release, he was sent back to prison for violating parole after being caught with cannabis and a stash of pornographic magazines.

In a town full of such men, the activities of Philip Garrido seemed simply to slip below the radar – despite the awful track record that has emerged since he and his wife, Nancy, were arrested and charged with 29 counts related to Jaycee Lee Dugard's abduction, imprisonment, and serial sexual abuse over 18 years (to which, it must be stressed, they have so far pleaded not guilty).

Court papers released yesterday from Garrido's 1977 trial for the kidnap and rape of a young woman in Nevada portray him as a dangerous sexual predator. During a psychiatric evaluation, he admitted to using LSD and cocaine as sexual stimulants and said that he would often masturbate in public, by the "side of schools, grammar schools and high schools, and in my own car while I was watching young females".

There is, however, no shortage of similar stories in Antioch. That is perhaps why, after Garrido was released in 1988, 10 years into a 50- year sentence, he and his wife were able to slip virtually unnoticed into the fabric of this community, which stretches for roughly four miles along the Sacramento River.

At first glance, Antioch may look like any other small American city. Its 100,000 residents are largely white and working class. Some work in industrial plants. Others are commuters, unable to afford the cost of living in the San Francisco Bay area. In keeping with most of California, about 10 per cent are unemployed.

Yet as police continue to investigate Garrido's past – and look into potential links to 10 murdered prostitutes and three missing girls (on Monday they announced the discovery of a bone fragment in his garden) – the city is being forced to confront a grisly truth: for reasons largely beyond its control, it has become a paedophiles' ghetto.

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At fault are laws governing America's treatment of sex offenders, which control where they are allowed to live and how much information the public should be given regarding their whereabouts. These laws were passed with the laudable intention of protecting children. But their actual effect is open to debate.

The most prominent is Megan's Law, which requires the public to be given access, usually via an internet site, to the names, addresses and "previous" of every man and woman convicted of a sexual offence. It is a well-intentioned exercise in open government. But in practice, critics say, it was introduced in such a way as to be of little help to anyone but the voyeuristic. "Thanks to political pressure, they made the criteria for including someone on the registry so wide that it has become totally ineffective," says Michael Risher, a lawyer for the American Civil Liberties Union.

"It doesn't just carry details of violent rapists but also people who, say, lost their temper during a road rage incident and flashed at someone, or an 18-year-old boy convicted of statutory rape for sleeping with his 17-year-old girlfriend."

In California, the Megan's Law website contains 90,000 entries. Given this extraordinary statistic, it isn't hard to see why the residents of Walnut Avenue – who had 121 other convicted sex offenders in their city to worry about – might have allowed a man with Garrido's dubious profile to pass largely ignored.

The second group of laws that make Antioch a magnet for paedophiles governs where they are allowed to live. In California, as in many states, voters have in recent years endorsed Jessica's Law, which bans paedophiles from residing within 2,000 feet of a school or a park where children regularly play.

This has driven sex offenders out of major cities and conurbations, where they have access to rehabilitation and treatment facilities, and into suburbs and secluded rural areas, where they don't. In some smaller cities, they have now become concentrated in such large numbers that parole and law enforcement officers are unable to properly vet them.

This may explain why local authorities never noticed that Garrido was apparently keeping the kidnapped Jaycee Lee Dugard and her two small children concealed in the elaborate series of sheds and tents in his back garden. Thanks to the influx of offenders to Antioch from major cities, they were simply too overstretched to do their job properly.

It may also explain – but not necessarily excuse – the fact that a police officer dispatched to investigate claims of children living in Garrido's garden in 2006 seemingly did not have either the time or the wherewithal to thoroughly research his suspect's background.

"If you look at maps that show where offenders are actually able to live under Jessica's Law, there's almost nowhere in the whole of Los Angeles and San Francisco where they can now legally settle," Mr Risher adds. "Everywhere is within 2,000 feet of a park or school. So they all end up in places like Antioch."

Even police admit that this leaves them struggling to cope. Daniel Terry, from the Contra Costa County Sheriff's Department, which oversees Antioch, has about 1,700 registered sex offenders in his jurisdiction. His station is responsible for about 350 of them, or "349 more than the number of detectives I have dedicated to monitoring these people."

Speaking to the Los Angeles Times this week, he said that the region's concentration of sex offenders was "significantly higher" than other areas in California and the rest of the United States."This is the reality. These people are walking amongst us everywhere." Adding to his woes are wider problems in the cheap parts of Antioch where Garrido and many other convicted sex offenders live. In the ramshackle area around Walnut Avenue, petty theft is rampant, drug abuse endemic – the favourite local tipples are crystal meth and crack cocaine – and lawns are littered with junk.

The city, which grew prosperous on the proceeds of the 1849 gold rush and then the steel mills and concrete factories that allowed it to ship the building blocks of San Francisco down-river during the early 20th century, is now among those caught in the storm of America's economic downturn.

Nearly 2,500 homes, roughly 5 per cent of the city's stock, are in foreclosure, with 699 new homes entering arrears last month. Property values have dropped 40 per cent in the past year and unemployment is soaring. Garrido's bungalow, a four-bedroom home built on a large plot of land between the wars, is worth just $100,000 (£61,800).

Against this background, and helped by laws that encourage ghetto-isation of sex offenders, it now seems that a man known as "Creepy Phil" by neighbours was able to take a little girl hostage, hold her for 18 years, father her two children, and even take them to community events, while barely raising an eyebrow.

In January, Zion Dutro, a convicted child rapist who lived on Alpha Way, not two miles from Walnut Avenue, appeared in court to plead not guilty to performing rape, sodomy and "lewd acts" on at least eight small girls. He faced 21 counts; his wife, a co-defendant, faced four.

In any other town, this kind of case would have sparked a mixture of shock and outrage that would be heard across the world. In Antioch, it merited no more than a few paragraphs in the local newspaper – a reaction which suggests that Jaycee Lee Dugard may not be the last grisly secret that the city reveals.

Sex offenders' register: Megan's and Jessica's laws

Megan's Law requires the public to be given details regarding the identity, whereabouts, and criminal record of convicted sex offenders living in their midst. It was named after the New Jersey schoolgirl Megan Kanka, who was kidnapped, raped and killed by a serial sex-offender in 1994. Today, it's being enacted to varying degrees in every US state.

Like any law passed in response to a public tragedy, the law has been dubbed knee-jerk by opponents, who say it encourages vigilantism and is an infringement on the civil liberties of ex-offenders. A study last year concluded that the law achieved no demonstrable reduction in child sex offences.

Jessica's Law prevents convicted sex offenders from living within a certain distance of schools, parks, and other areas where children gather. It was first adopted in Florida in 2005 after nine-year-old Jessica Lunsford was snatched from her home, before being raped and murdered by a convicted paedophile. Today, a version is in force in 42 US states. Critics say it has made major cities off-limits to offenders, forcing some to declare themselves homeless, and ghettoising others. There is little evidence that it works. Many police forces say it has the opposite effect: stretching resources and doing nothing to prevent paedophiles travelling to commit crimes.