By Andy Gallacher
BBC Miami correspondent
Megan Kanka was just seven years old when she was killed in July 1994. The little girl was lured into a neighbour's house in a quiet suburb of New Jersey on the promise of seeing a puppy.
Megan's death soon prompted a change in US law
Shortly afterwards she was raped and murdered just metres from her home.
Megan's parents, Richard and Maureen, had thought that their daughter was safe.
They did not know that a convicted child molester had moved into their street.
Jesse Timmendequas had already served six years in jail for aggravated assault and attempted sexual assault on another child.
In the weeks that followed her death, almost 500,000 people signed a petition demanding a change in the law.
Within an unprecedented three months the State of New Jersey passed Megan's Law.
Since then it has been adopted in some form by all 50 states, and Megan's mother Maureen Kanka says it has given real power to concerned parents.
"It's been very effective. For the public it's been a tremendous tool because it provides an awareness that they otherwise would not have had," she said.
"It has definitely heightened and highlighted the awareness of child sexual abuse and made parents more alert and aware."
Tracking sex offenders
Fifteen states now list offenders' details on the internet. All concerned residents have to do is enter their postcode and they can find out whether child sex offenders have moved into their area.
In some states the authorities send out warning e-mails, in others offenders can be forced to display signs in their windows.
Supporters of Megan's Law point to cases where sex offenders have been discovered working as youth counsellors, in amusement parks or other areas where they have come into close contact with children.
Maureen Kanka says Megan's Law has made parents more aware
But not everyone agrees that Megan's Law is the best way of tackling the re-integration of child sex offenders into society.
Jack Levin is the director of the Brudnick Centre on Violence at Northeastern University in Boston. He has published several books on murder.
"It will push people back into a life of crime," he said. "It has made it impossible for these sex offenders to be integrated into the community.
"It's like this feel good law that makes you believe you are safe. It makes it more likely that they will recidivate [reoffend]. There are many sex offenders who refuse to register."
Mr Levin's argument has been backed up by research. Past studies have shown that fewer paedophiles register, as they are required to by law, than they do in the United Kingdom.
Critics also point out that most cases of child abuse happen within the family and may go unreported because of the implications of Megan's law.
Other experts also point to the danger of children being identified because of the law. There is no provision under Megan's Law to protect the identity of victims.
Mr Levin says there is a better way, what he calls a two-strikes-and-out approach.
"They shouldn't be in anybody's neighbourhood. They should be incarcerated for the rest of their lives.
"If you're a repeat sex offender who is dangerous to the public, two strikes and you're never out again."
That is something Mrs Kanka agrees with, but for her it is a simpler issue.
"We did it because it is something that should have been there for her [Megan] and it wasn't," she said.
"Nobody has come up with a better way of dealing with it and as long as these offenders are out there we have a right to know who they are."