In many ways, the worst criminals are sex offenders. These people are mostly distorted in the mind and their victims are often children. Even hardcore thugs pose no direct danger to people who get out of their way, much less children. On the contrary, sex offenders search out the easiest and most vulnerable, regardless of age. Fortunately, their threat and danger is well publicized and legal measures are in place to keep them in check.
Convicted sex offenders are required by law to register their whereabouts and activities with the authorities even after they have served out their sentences. It is necessary to keep track of them as such people are prone to repeat offense. Statistics vary over different states but the recidivism rate within 3 years of released sex offenders in general is around 3% and 15 to 20% of their repeat offenses are actually committed while on probation or parole.
Another stringent restriction on convicted sex offenders in many states is their residency. Such people are forbidden from staying within certain distances, say 1000 or 2000 feet, of child-centered amenities such as schools, playground, childcare centers or even bus stops. Sex offender registry are also made available for public viewing by many jurisdictions through government websites, notice boards, mail and other media.