The NICS is administered by the Federal Bureau of Investigations (FBI).  A NICS check includes a check of three databases maintained by the FBI, including the:

• Interstate Identification Index (III), a database of criminal history record information,
• National Crime Information Center (NCIC) which includes information on persons subject to civil protection orders and arrest warrants, and
• NICS Index which includes the information contributed by federal and state agencies identifying persons prohibited from possessing firearms who are not included in the III or NCIC, such as persons with a prohibiting mental health history or who are illegal or unlawful aliens.

If a NICS check identifies a person as falling within a prohibited category, the FBI advises the FFL that the transfer is "denied." Individuals can appeal denials and seek the correction of the information in the FBI databases by either applying to the FBI or the federal or state agency that contributed the information to the FBI.