He was wanted on a felony arrest warrant in Florida. He had a 1993 conviction in Frederick County for soliciting a teenage boy and faced charges in Charles County last year after a boy accused him of molestation, court records show.

When asked, Thomas Richard Koucky admitted to police that he had little control over his desire for boys and may have molested 300 of them. "If I walk out this door and a young boy is standing there and I'm around, it's all over," he said in comments quoted in a court report.

Yet when Koucky, 41, was arrested in Arlington County last month, he did not appear on Internet sex offender registries accessible to residents in Maryland, where he worked; Virginia, where he lived; or the District, where he used to live -- even though those registries were put in place to warn people about offenders like him, officials said.

"We can only make public information that we are legally allowed to put out there," said Rosa Cruz, deputy director of public information for the Maryland Department of Public Safety and Correctional Services. "If someone is charged and the charges are not [prosecuted], we can't put their name out there. We try to do the best we can. . . . It's frustrating, though, when you hear of cases that don't go the way the community wants them to go."

National child safety advocates say the case underscores a larger problem with the registries, which they say are riddled with loopholes and rely on the sex offenders to provide the necessary information.

A recent study showed that as many as 40 percent of sex offenders who register give a fake address, said John Walsh, host of the "America's Most Wanted" television program.

"Thirty percent don't register at all," Walsh added. "They are on probation and parole, and that is a privilege. They should be mandated to register before they get out of prison. There is too little money and training behind the legislation, and that has allowed convicted child molesters to beat [detection] time and time again."

Koucky's case came to light after he was charged in Arlington with molesting a 17-year-old youth -- and then released on $3,000 bond. He fled to Guatemala but was arrested there and returned to Arlington on Friday. He faces charges in the alleged molestation and failure to appear for a court hearing.

In his case, a series of loopholes in the requirements helped him repeatedly escape mention. He was convicted in Frederick four years before Maryland required offenders to register and nine years before the state's registry went on the Web.

He later moved to the District, which had a registration requirement but does not give information on crimes such as his to the public.

He was arrested in Charles County, but because the case was not prosecuted, there was no requirement that he register.

In Virginia, he's registered, but because his crime was not deemed violent, he was not included on a site accessible to all residents. The state has two registry classifications: violent sex offenders, whose names are posted on the Web site, and other sex offenders, whose names are not, officials said. Koucky's name was included on a separate listing available to day-care centers, schools, foster or group homes and similar facilities in Virginia.

But Koucky worked in real estate in Southern Maryland.

As such, Maryland officials said he should have been listed on the state's sex offender registry. In fact, he was charged in 2003 with a probation violation for failing to notify officials of his address after moving to Virginia from the District. According to court records, Koucky was sentenced to four years for the violation, but all but six months of the sentence was suspended. He was released after four months and returned to his home in Virginia after asking to have his probation supervision transferred, officials said.

Still, no one told the proper authorities to add Koucky's name to the public registry, officials said.

"When it was transferred . . . we in Maryland had no more contact with him because legally we couldn't," said Mark Vernarelli, director of public information for the Maryland Department of Public Safety and Correctional Services, even though Koucky continued to work in Maryland.

Lawyer Carolyn Atwell-Davis of the National Center for Missing and Exploited Children in Alexandria said such communication failures often defeat the purpose of such registries. Her center also recommends that states toughen their laws to keep offenders from falling through the cracks.

"Every state's registration program should require offenders to register before they are released from prison on parole or given probation," she said. "Also, the penalty for failure to register should be a felony. . . . And failure to register or re-register should be an automatic revocation of probation or parole."

The registries were implemented to help law enforcement officials keep track of offenders and to allow others to identify predators. All but 15 states make information on at least some offenders available on the Internet; others make it available from police. The FBI maintains its own National Sex Offender Registry, which is not accessible to the public.

The online listings in Virginia, Maryland and the District provide registrants' names, addresses, photos, offenses, status and last-known employment or school. Offenders who are Virginia residents or students or who work in the state are required to register, authorities said.

The Maryland registry, which was implemented in 1997 and went online in 2002, allows information about any convicted sex offender to be viewed. There are 3,683 on the list. Virginia's Sex Offender and Crimes Against Minors Registry, which lists more than 12,500 offenders, allows public access via the Internet only to information about those convicted of violent offenses. The District site, operated by D.C. police, lists information on 549 offenders convicted of felonies and crimes in which the victim was younger than 12, police said. A D.C. police sergeant in charge of the registry did not return calls for information.

Virginia officials admit their registry's shortcomings right on the Web site. "The data contained in sex offender registrations may be primarily based upon information furnished by a convicted sex offender and not substantiated by source criminal record documents," such as fingerprints, the site says. Sites in Maryland and the District have similar disclaimers.

Despite the disclaimers, Virginia officials said they work hard to track offenders. Violent offenders are required to re-register every 90 days. State police send a certified letter to their listed address, and they must sign and return the letter along with a thumbprint. If the letter is returned as undeliverable, police investigate, said Debbie Mann, manager of the registry. Nonviolent sex offenders must re-register annually for 10 years, she said.

Tougher laws against nonviolent offenders, as well as increased vigilance, are essential, said Walsh, who became a child safety advocate after his son was abducted and killed.

"As the father of a murdered child and as a guy who has hunted thousands of pedophiles, I can tell you, not registering nonviolent sexual predators is" nonsense, he said. "That would be the guy who is moving down the block from you. He may kill the kid who says, 'I'm going to tell.' If he has abused a child and ruined a child's life, as these guys do, the public should be able to find out about him."

Staff researcher Bobbye Pratt and staff writer Arthur Santana contributed to this report.

"Every state's registration program should require offenders to register before they are released from prison on parole or given probation," says Carolyn Atwell-Davis of the National Center for Missing and Exploited Children.