Sex offender registries in the United States exist at both the federal and state levels. Registries contain information about persons convicted of sexual offenses for law enforcement and public notification purposes. All 50 states and the District of Columbia maintain sex offender registries that are open to the public via websites, although information on some offenders is visible to law enforcement only. Public disclosure of offender information varies between the states depending on offenders designated tier, which may also vary from state to state, or risk assessment result. According to NCMEC, as of 2016 there were 859,500 registered sex offenders in United States.