The addresses for one out of every four offenders in Montana's sexual and violent offender registry are unverified and possibly unreliable, casting doubt on the credibility of the registry used by everyone from concerned parents checking out new neighbors to house hunters in search of a safe neighborhood.

A new audit found that 26 percent of the offenders in the state's database have not returned a required address verification letter even though not doing so puts them at risk of going back to prison. The report found that the state never notified members of the public or local police checking the list that many of the addresses are not verified.

The backers of the advocacy group that pushed Congress to adopt Megan's Law in the mid-1990s, which requires states to make information on sex offenders available to the public, said the biggest problem all along with the registries has been maintaining accurate data.

Most states notify the public with websites like Montana uses, and many have struggled over the years to keep the information current, said Maureen Kanka, whose daughter Megan was killed in 1994 by a sex offender — a crime that helped launch the push for public registries.

"Unfortunately if law enforcement drops the ball you are going to run into problems like this. That is one of the biggest problems, the lack of following through," said Kanka. "That is probably the most common problem they have with the registries is making sure they are kept up to date. That is not unheard of."

Montana officials pointed out that an unverified address in its system does not necessarily mean the offender has moved elsewhere and could be a threat to unsuspecting neighbors. The Montana Department of Justice, which tracks roughly 5,000 offenders in the state, said some have simply failed to return the address verification letter required by law.

In nearly a third of the cases, the offenders are back in jail and the managers of the database were never notified. And the auditors found seven cases where dead offenders were still listed as active in the registry.

"When members of the public access the Web site or law enforcement queries data, thy will not be aware of the offender's failure to verify their registration," the auditors wrote in a new report. "Therefore, they may not be aware of the offender's actual location."

But in many instances, the legislative audit revealed, authorities aren't sure where the offenders are living.

The Department of Justice said that it is making some changes.

The agency plans to flag unverified offenders so those looking at the website are aware of the situation. Additionally, it plans to send local police frequent reports on unverified offenders so officers can check on unverified offenders, and track them down in need be.

Auditors reported that agency managers initially resisted flagging unverified offenders because it "would lead the public and law enforcement to question the data in the registry since such a large number have not verified their registration."

Mike Batista, a Department of Justice administrator, said the improvements will be made. But overall, he said "by and large" the public can rely on the website that provides a map of offender locations.

"I think they can rely on a lot of the information," Batista said. "It's a challenging program in terms of being able to constantly know on any given day where an offender resides."

Kanka said the public should help police, noting it is impossible for authorities to constantly monitor the offenders.

"There is nothing wrong with being diligent and being watchful and making police aware if you notice an offender has moved and is not living where they are supposed to anymore," Kanka said. "The registries are only as good as what we put into them."