BOSTON -- The sex offender registry is the state's way of letting residents know about potentially dangerous sex offenders. But what about the sex offenders that are missing?

Team 5 Investigates' Sean Kelly said the missing sex offenders are called "Level Zeros" by police.

Nathan Robinson is a convicted rapist. But for the past two decades, he has been unregistered and unclassified as a sex offender. His neighbors had no way of knowing his past until last February when prosecutors charged him with raping a Brockton woman.

Until then, Robinson was listed as a Level Zero violator, or unclassified sex offender. Officials said there are thousands of unclassified sex offenders like Robinson who were released from prison before the sex offender registry became law. Without a classification, they are under the radar of police and the public, and no one knows how dangerous they are.

"These people don't want to be found," said State Police Lt. Kevin Horton.

Horton runs the State Police Violent Fugitive Task Force. The task force hunts for the most wanted criminals, including sex offenders who've gone into hiding.

"I'd be worried about all of them. We don't know at this point what the Level Zero is going to be classified. But you have a good idea if you look at this criminal history," Horton said.

Team 5 Investigates found that as of a month ago, there are almost 2,000 Level Zero violators unaccounted for in Massachusetts alone. Some could be dead, out of state or in residential neighborhoods.

"You could have a sex offender living next door or down the street or across the street from your child's bus stop. We should have that info," said victims' advocate Laurie Myers. "We are far from a perfect system."

Officials have tracked down some of the missing sex offenders. Many of them have since been classified as Level 3 offenders -- the most dangerous. Until they were caught, their names, faces and addresses were not posted on the Sex Offender Registry Board's Web site.

State police aren't just looking for the Level Zero offenders. They're also looking for classified offenders who haven't re-registered, as they are required to do annually. Team 5 Investigates found that almost 200 of them are the worst of the worst -- Level 3s -- and they could be anywhere.

Catching unregistered sex offenders does not mean they will be punished, victims said.

"Nothing can give back to my kids what was taken from them. We're all changed forever," said one mother, who wished to remain anonymous.

Glenn Wheeler, the man who molested the woman's three children didn't have to serve any extra time, even though he didn't register numerous times.

"What happened to protecting the victim? What happened to justice?" the mother asked.

John Manning, of Fall River, one of the state police's most wanted, failed to register four times. After the first three, police caught Manning, but the judicial system let him go. After Manning's arrest for not registering for a fourth time, he was sentenced to 2 ½ years in jail.

"It seems like when people violate, it isn't taken seriously in the courts, or anywhere else for that matter," Myers said.

District attorneys told Team 5 that they are at the mercy of judges when it comes to sentencing. Officials hope a new law will better protect residents by making sure sex offenders are registered before they ever leave prison.