(CBS) — Illinois puts the home addresses of more than 31,000 sex offenders on a website so residents can see who lives in their neighborhoods.

CBS 2 found that many of those addresses are bogus.

How could so many slip through the cracks?

In one case, CBS 2’s Megan Hickey found a mother of young children living in a Palatine apartment building, who had no idea she had a registered sex offender as a neighbor.

That person registered with a Chicago address which doesn’t exist. In addition, he used the 60606 zip code, which is the Willis Tower.

Another sex offender who actually lives on the Northwest Side, only a few hundred feet from an elementary school, also registered with that 60606 zip. He is convicted of sexually abusing a 16-year old.

Another offender registered at 69 W. Washington St., which is the administrative building for Cook County. It’s also home to the Cook County State’s Attorney’s office. A security guard there confirmed there are no residential properties at that address.

“This is all office building. That’s all it is. Nobody lives here. No, that makes no sense at all,” the guard said.

One offender got away with registering a P.O. Box. Another offender lives on Hatred Street in Evanston–which is not a real street.

So how are these addresses making the cut?

The Attorney General’s office referred inquiries to Illinois State Police.

Illinois State Police acknowledged they maintain the database, but said the list is only as good as the data entered by local law enforcement, the county sheriff or the Illinois Department of Corrections.

ISP goes as far as putting a disclaimer on its website warning that “ISP makes no representation, express or implied, that the information contained on the registry is accurate.”

Laura Rogers is director of the Sex Offender Sentencing, Monitoring, Apprehending, Registering, and Tracking (SMART) Office, within the U.S. Department of Justice.

“A P.O. Box would never be an acceptable address for a sex offender,” said Rogers, who added that Illinois hasn’t fully implemented federal guidelines for sex offender notification and registration.

That’s why the state has received a funding penalty every year since July 2011.

Some of the solutions to verifying addresses are pretty simple.

“We have so many great technological features these days. There’s no reason why you can’t just Google that house address to see that it’s a house address versus a post office box,” Rogers added.

One of the 17 states that does follow the federal guidelines is Ohio. That state’s attorney general, Dave Yost, had a few critiques for Illinois.

“Illinois lacks a way to be able to consistently be able to notify people by email, or to update people if interested,” Yost said.

He said notification and address verification have been priorities for the Buckeye State. They send postcards to verify that addresses are real.

“It’s not hard to send out a batch of postcards and see what comes back,” Yost said. “This stuff is not hard to do. It’s not especially expensive. I would just urge people to buckle down and get it done.”

Meanwhile, residents are asking who else could be lying about where they live.

“It’s scary, you know? We have to take precautions,” the mom from Palatine said.

Scariest of all: no one seems to know who’s accountable for all these wrong addresses.

A spokesperson for the Cook County Sheriff’s Office says addresses in their jurisdiction — unincorporated Cook County and the Village of Ford Heights — are physically checked to make sure offenders are in compliance. Registered sex offenders also come to the Sheriff’s Police Department annually on Halloween to update their information.

A spokesperson for the Illinois Department of Corrections also said if the offender has been discharged from IDOC custody, it is the responsibility of local law enforcement to verify the address. The Parole Office verifies the addresses for those on supervision.

This story has been corrected to reflect that the Illinois State Police commented on the inaccuracies in the sex offender registry.

Megan Hickey