TAMPA, Fla. -- A convicted rapist who had failed to register as a sex offender is accused of strangling a 13-year-old girl near Tampa. Three weeks earlier and 100 miles north, a registered sex offender who did not tell police he had moved is charged with abducting and killing a 9-year-old girl.
While it is hard to say whether better tracking could have prevented the tragedies, experts say such cases around the country underscore the difficulty authorities face in keeping tabs on convicted sex offenders who are required by law to report their whereabouts.
National surveys have shown that about one-fourth of the more than 500,000 sex offenders who are on the streets have moved, failed to report new addresses to police and eluded detection.
"There are so many sex offenders and so little resources law enforcement has to track them down," said Laura A. Ahearn, executive director of Parents for Megan's Law, an advocacy group.
All 50 states have passed laws requiring sex offenders to register with law enforcement since Congress in 1996 enacted Megan's Law, which calls for warning communities of sex offenders in their midst. That law was inspired by the case of 7-year-old Megan Kanka, a New Jersey girl raped and killed by a child molester living across the street.
But police say limited manpower prevents them from doing much more than making random spot checks to see whether offenders live at their listed addresses. Officers often must rely on neighbors to report departures, particularly in states where offenders are only required to register by mail once a year.
Convicted sex offenders know all too well that they can lose their jobs and their homes and suffer other retaliation if the neighbors find out about their past.
"Many sex offenders don't register -- notwithstanding the prospect of going back to prison if they're caught -- because they know the consequences," said Jack Levin, a criminologist at Northeastern University in Boston.
States have acted to tighten the laws in recent years, changing administrative procedures, increasing penalties for those who fail to register and requiring them to report more often. In 2004 alone, states passed more than 80 laws to keep better tabs on sex offenders.
States "did a good job of getting these registries up and running in the first place, and they have had to revisit them and see where they can improve them," said Blake Harrison, a policy specialist with the National Conference of State Legislatures. "There's a lot of pressure from the public" to know where offenders are.
Ahearn said legislative and law enforcement efforts must focus not only on making sure offenders are registered but on better monitoring, possibly using electronic bracelets or satellite global positioning systems.
Florida is widely considered among the best at maintaining its registry and monitoring sex offenders. Still, a recent Miami Herald study showed that the state lost track of at least 1,800 sex offenders in the month before 9-year-old Jessica Lunsford was slain in the town of Homosassa in February.
On Tuesday, the Florida House unanimously passed a bill called the Jessica Lunsford Act that would set a mandatory sentence of 25 years to life for people convicted of molesting children under 12. Once released, such offenders would have to wear a global positioning system device. The measure goes next to the Senate.
Some investigators say no amount of monitoring is going to stop a sex offender who is bent on committing crimes again.
John Evander Couey -- the man accused of taking Jessica from her bed, sexually assaulting her, killing her and burying her -- had been convicted in 1991 of fondling himself in front of a girl. He had left his registered address to stay at his half-sister's trailer, 150 yards from Jessica's house, without authorities' knowledge.
"If you have a bona fide child molester, the only thing that is going to stop him is a steel door," said Hillsborough County Sheriff David Gee. "They are going to find ways."
David Onstott, the man accused in the slaying of 13-year-old Sarah Lunde earlier this month, was convicted of a sex crime in 1995. In March, he was arrested and charged with failing to register. Authorities say he strangled the girl and dumped her body in a pond after getting out of jail on bail.
Mark Ober, the region's chief prosecutor, said the best way to prevent such crimes is for neighbors and parents to be more vigilant.
"People need to be their brother's keeper," Ober said. "They need to take care of their children and watch out for their neighbors. This has to stop."
Associated Press writer John Nolan contributed to this report.