The abduction and killing of 9-year-old Jessica Marie Lunsford -- allegedly by a convicted sex offender who lived nearby and had failed to register with authorities -- is a grim reminder of the flaws in the country's tracking system, victims' advocates say.
John Evander Couey, 46, was formally charged today with capital murder in the abduction and death by asphyxiation of Jessica. He had failed to register his change of address as required for a sex offender, authorities say.
There are more than 400,000 registered sex offenders in the United States -- convicted criminals whose fingerprints, names and addresses have been recorded on official lists. But victims' advocates say almost 25 percent of them have slipped through the cracks, and authorities no longer know where they live.
"The problem is sex offenders are responsible for their own compliance," said Donna Coleman, president of Children's Advocacy Alliance. "So you have felons that are responsible for complying to register, which is crazy because they're not exactly your top-notch citizens."
States have required sex offenders to register since 1996, when Congress passed Megan's Law -- named after 7-year-old Megan Kanka, who was raped and murdered by a twice-convicted child molester who lived across the street.
But even with the legislation, every state is free to decide which criminals must be registered and how much personal information will be made publicly available. Some states actively distribute the names while others require public inquiries.
States' methods for tracking known offenders also vary. Some conduct periodic searches of registered addresses while others require offenders to submit to polygraphs as part of their parole.
In Texas, for instance, authorities have used global positioning satellite systems with transmitters attached to offenders in an effort to track their movement.
Additional funding might improve tracking of sex offenders, but legal issues also present difficulties. Sex offenders have rights, and there are limits to the lengths of their incarcerations.
"The real issue is how do you follow up on 400,000 offenders?" said Ernie Allen, president of the National Center for Missing and Exploited Children. "They're 93,000 in California alone, almost 26,000 in Florida. So we know it's a massive problem. But it's one that for the safety of the community, we have to build better systems and keep better track of who these people are and what they're doing."
ABC News' Bob Woodruff filed this report for "World News Tonight."