Several thousand sex offenders required by law to report their whereabouts have gone missing and can't be found, according to officials from several states.

Michigan alone cannot locate 1,091 of the 29,500 people recorded in a federally mandated state sex offender registry as of June 1, an official there told Fox News.

"We seem to have a problem just being able to track them once they leave the state. If they leave — we can't predict what human behavior is," said Charlotte Marshall, sex offender registration analyst for the Michigan State Police. "Without putting a tracking device on them, we don't know where they're going to end up."

And the number could actually be even higher. "These are only the ones we know about," Marshall conceded.

"We are all facing the same issues" in the states, Marshall said. "We realize we aren't alone — all the other states are going through the same thing."

The Georgia Bureau of Investigation has lost about 160 out of about 6,000 sex offenders, officials there said. But that figure, too, could be much higher.

Some 1,200-1,800 of the letters the GBI sends out each year to sex offenders are returned to sender, indicating the offenders may have moved away without leaving a forwarding address.

The problem exists even in states that don't have a large number of offenders.

Between 100-120 offenders out of 2,030 adults in Idaho failed to register by July 1, said registry administrator Dawn Peck.

"We do have a number of offenders who are not compliant," said Peck, who said the establishment of the state offender registry has nonetheless gone fairly well. "I'm sure there's always something that could be done somewhat differently, but I think overall, the consensus is, it's pretty successful.”

Federal law mandates each state to establish sex-offender registries that allow residents to find out if there are convicted offenders living nearby their homes, or where their children go to school. But only 30 or so states even have their registries up and running.

Each state runs their registry differently. Some provide only public access to information on violent sexual offenders, while others make residents fill out a form to check up on an offender.

But in all states, federal law requires the offenders to check in with local law enforcement whenever they move to another area or change jobs.

States are working together to track the movements of the felons to make sure they are registered. At least 25 states have enacted the Interstate Compact on Adult Supervision, which sets up uniform guidelines for how states send and receive information on offenders.

Some states focus on only a small number of those offenders. In New York, for instance, police only put "level 3," or "high-risk" violent sexual offenders online for the public to see. As of July 18, there were 14,404 registered sex offenders of all kinds in New York.

"It is a work in progress," said police spokesman Scott Steinhardt, who said he could not say how many offenders have gone missing.

Federal law also requires all states to do even more with their registries by Oct. 27, 2002, or run the risk of losing grant money to fund the programs.