CBS News correspondent Ben Tracy reports Gardner, 30, is a known predator. In 2000, he pleaded guilty to molesting his 13-year-old neighbor. His psychiatrist said he was "a continued danger to underage girls in the community" and recommended a 30-year sentence.
However, prosecutors made a plea deal so the victim didn't have to testify. Gardner served just five years.
He did wear a GPS monitoring device until 2008, but since then investigators think he may be involved in the disappearance of a 14-year-old girl last year and may have attacked a 22-year-old woman in December. Convicted sex offender Jake Goldenflame says without therapy, prison terms are useless.
"It's wishful thinking to believe that simply by locking someone up for a number of years that's going to magically transform him when he comes out," Goldenflame said.
Experts say there are not enough parole officers to monitor the more than 700,000 registered sex offenders in the United States. At least 100,000 may not even be.
"The sex offender registry provides this false sense of security we are monitoring and doing something with the sex offenders out there," said Robin Sax, a former Los Angeles county prosecutor.
And monitoring doesn't always work. Even with home inspections, Phillip Garrido was still able to hidein his backyard for 18 years. Last November in Cleveland, in the house of another registered sex offender being monitored by police.
If convicted, John Gardner will now spend the rest of his life behind bars -- but some say if the system were tougher he'd already be there.