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Jessica Lunsford (search) and Jetseta Gage (search): two young girls recently kidnapped, molested and killed, allegedly by convicted, registered sex offenders who had served time and were back in the community.

Criminologists say it’s all too frequent that the perpetrator in such cases is a pathological sexual predator, as is true of Jessica’s alleged killer in Florida, John Evander Couey (search), and Roger Paul Bentley (search), who Iowa police say murdered Jetseta.

“It happens all the time,” said Louis B. Schlesinger, a forensic psychologist specializing in criminal behavior and sex crimes at John J. College of Criminal Justice (search) in New York. “The dangerous ones have a high recidivism rate.”

According to data from the FBI’s National Crime Information Center (search), there are 381,967 entries for sex offenders in the NCIC Sex Offender Registration File — though not all states require sex offenders to be registered in the same way and some offenders are entered into the database for more than one state.

The Jessica Lunsford case so outraged her Florida community that a state representative, Charles Dean, said he’s introducing a bill called the "Jessica Lunsford Act" that would, among other things, require convicted sex offenders to wear electronic tracking devices.

“It’s a matter of us doing the job right. We need to find the loopholes, find the cracks,” Dean told FOX News.

The reason many convicted sex offenders go out and molest more children, say sociologists and criminologists, is similar to why alcoholics continue to drink.

“Their sexual preference is for children. They have a compulsion to molest children,” said Keith F. Durkin, a criminologist at Ohio Northern University (search) and an expert in the study of pedophilia. “Many, if not all, will molest children until the day they die. They’re dangerous and they’re going to reoffend.”

But there aren’t accurate numbers about the rate of recidivism among child molesters, since many of their repeat offenses go unreported.

Not only are they almost certain to continue sexually abusing children, but some eventually kill their young victims — more often than not for the purpose of keeping them quiet.

“Usually it’s to cover up the crime so the victim won’t say who he is,” Schlesinger said.

Nine-year-old Jessica Lunsford was snatched from her bedroom last month by a drug-addled Couey, police said. Couey was staying with relatives in a trailer just across from where Jessica lived with her father and grandparents in Citrus County (search), Fla. After holding her for a few days and sexually assaulting her, police said Couey, 46, killed the little girl and buried her body only a few hundred yards from her house. The coroner ruled that Jessica died by asphyxiation.

Bentley, 37, is accused of abducting 10-year-old Jetseta from her home in Cedar Rapids (search), Iowa, last week and killing her. Her cause of death also was found to be asphyxiation, and authorities said there was evidence the mentally challenged girl had been molested. Bentley’s brother James Bentley, whom Jetseta’s mother dated five years ago, also allegedly sexually abused Jetseta and is in jail awaiting trial on those charges.

It’s unlikely that there’s any sort of chain reaction element to such sexual crimes against children, according to experts.

“The copycat value and the deterrent of being caught have minimal impact,” said Jeffrey Ian Ross, a criminologist at the University of Baltimore (search). “It’s not a crime in which reason prevails.”

But a number of sex offenders do know they can’t be trusted around kids. Couey reportedly was so aware of his problem that he’d pleaded for help in the past, saying he was a danger to children because he couldn’t stop himself from sexually abusing them. That compulsion is what makes it next to impossible to “cure” chronic child molesters.

“They’re basically untreatable,” Schlesinger said. “They’re predatory, compulsive, repetitive offenders. These are very dangerous people, aroused by children. That’s part of their sexuality. It’s very, very difficult to change that.”

In spite of that reality, many still are only serving fractions of their sentences — which often are light to begin with.

“The bottom line is that almost all these offenders will get out because they don’t have any laws barring them from getting out,” Schlesinger said. “They’re allowed back in the community because the court system is following the law.”

That’s why parents and others in the community frequently are helpless to bring about any real change to prevent children from falling prey to molesters living in the area.

“The public is getting increasingly upset when this happens,” Ross said, “but they feel their hands are tied.” People can take action by writing letters to local newspapers and Congress, he said.

Ernie Allen, president of the National Center for Missing and Exploited Children (search), said the existing legislation is a good start but needs to be beefed up — especially since authorities usually must rely on the offenders themselves to notify them of who and where they are.

Megan’s Law (search) requires all 50 states to register sex offenders but leaves the details up to the individual states. The burden generally is on the offenders to register themselves and alert officials when they move. The sex offender registries are available online and provide basic information about and photographs of those in the database.

“States should be more active in notification,” Allen told FOX News. “We think the Web sites are great, but that’s not enough. Megan’s Law ought to be strengthened in every state.”

In the meantime, he said, parents need to take advantage of the resources already available to them and talk with their children so they know where they are and who they’re with.

“Every parent needs to go to these Web sites and find out who the registered sex offenders are in their community,” Allen told FOX.

Ross believes cases such as Jessica’s and Jetseta’s could have an impact on legislation if there’s enough public reaction to elicit federal, rather than state, sponsorship of a tougher law.

He predicted that at the very least, the conditions for releasing and paroling jailed sex offenders will tighten as a result of the murders of Jessica, Jetseta and other children in similar situations.

“There may well be a strong enough backlash,” Ross said. “These kinds of cases have enormous repercussions. They tug at the heartstrings of most Americans who have children.”

But others are more skeptical that any real progress will be made in controlling the problem of convicted sex offenders committing more crimes against children.

“There will be a call for increased monitoring that will then fall by the wayside,” said university criminologist Durkin, who believes convicted child molesters should be placed in special communities just for them. “But we have to take a close look at these repetitive sex offenders as a country. When we’re putting them in the community, we’re putting children at risk.”

FOX News' Heather Nauert and Martha MacCallum contributed to this report.