BOSTON -- Checking the online Sex Offender Registry Board could be giving residents a false sense of security, Team 5 Investigates found. So, how many registered Level 3 and Level 2 sex offenders live in each town?

Team 5 Investigates' Sean Kelly picked two Massachusetts communities with high populations of the most dangerous registered sex offenders in Massachusetts -- Worcester and Dorchester. Combined, they have more than 200 Level 3 sex offenders living or working there.

"I think having this information for communities, I think we need it," said victim advocate Laurie Myers.

Myers co-founded a victims' rights group called Community Voices. The group wants full disclosure of the names, faces and addresses for all sex offenders.

"We've said a million times that anonymity is a sex offender's best weapon," Myers said.

Myers said parents could be putting too much stock in the online registry.

"This is what we have left, and we have to build on that," Myers said.

Team 5 Investigates used the registry to see which offenders live close to schools and found that too often "close" on the map isn't close at all in person. That is one reason why experts warn parents against using the sex offender registry as their only prevention tool.

"My problem is that piece of mind is founded upon, to some degree, a false sense of security. Sex offenders could and would more likely come into areas where they're not known to commit their offense," said David Medoff, a forensic psychologist at Suffolk University.

In places like Wellesley, Needham and 119 other Massachusetts cities, there are no registered Level 3 sex offenders living there.

But forensic psychologists said parents should not let their guard down over that statistic.

"Offenders will mostly likely go to a different area, zip code, or school, or a YMCA where they're not known," Medoff said.

According to Robert Baker, director of operations for the Massachusetts Sex Offender Registry Board, for every 100 sexual offenses committed, 30 to 40 get reported and 24 to 28 victims in those cases will identify the offender. Some of the victims do not identify the offender.

"If most offenses are not detected, then there are many more people out there who are not registered than who are," Medoff said.

A local mother said a friend of the family with no criminal history molested all three of her children

"From the very first time our 5-year-old went there, he was molesting her," the mother said. "If you let your kids out of your sight, they are at risk."

The mother said mapping sex offenders and studying the registry do not offer the perfect shield. Advocates agree but insisted it's one important line of defense.

"Parents need to take that extra step. They're the ones that can really protect their children," Myers said.