WASHINGTON - Sex offender registries are often inaccurate and incomplete, undermining public knowledge about some of the nation's most reviled criminals, Justice Department investigators warn.

The national sex registry is missing information on 22 percent of state-level sex offenders, the federal investigators found. Driver's license information, Social Security numbers and basic addresses are regularly absent, potentially leaving neighbors and police alike in the dark.

"As a result, members of the public will not have the information they need to assess the threat posed by sex offenders in their communities," the Justice Department's Office of Inspector General cautioned.

The investigators aren't completely critical in their new 110-page report assessing progress in tracking sex offenders. They praise, for instance, the U.S. Marshals Service for increasing investigations and arrests of fugitives. The Marshals Service conducted 2,621 fugitive sex-offender investigations last year, up from 390 in 2004.

However, even as sex registry information becomes more widely accessible via the Internet, investigators sound alarms about the databases used to monitor the nation's 644,000 registered sex offenders. The concerns coincide with more fundamental questions about whether the stigmatizing registries go too far.

An advocacy group called Texas Voices is trying to change that state's sex-offender registration requirements so that they don't cover so many crimes. Other critics contend that Congress lacks the constitutional authority to require sex offenders to register anew when they move into new states.

Multiple registries have sprung up since the mid-1990s. The FBI maintains the National Sex Offenders Registry, and all 50 states maintain their own registries, though they differ.

Florida, for instance, allows the public to search for e-mail addresses used by registered sex offenders. North Carolina locates sex offenders by longitude and latitude. California allows searches by proximity to parks or schools.

California leads the nation in registered sex offenders, with about 114,000. This is more than twice the number of sex offenders registered in Texas or Florida, and 10 times the number registered in North Carolina.

Investigators found that some state registries aren't yet compatible with the national FBI registry. Some state files were rejected because they lacked information that the national database requires.