that s who you are buying from, contacts nics by phone or internet. nics, which is operated by the fbi, references three data bases, the nics index, national criminal background check system, interstate identification index and national crime information center, the ncic. if none of these data bases finds anything wrong, the gun sale goes ahead. if the data base finds a problem, federal agents have three business days to determine whether to deny that gun sale. if nothing comes up after three days, they don t hold the sale. the sale continues. what could lead to a denial? if a potential buyer has a criminal history or drug use, mental health problems, is not a u.s. citizen, has a history of domestic violence or has been dishonorably discharged from the military, could hold it up. that should have prevented devin kelly, the gun man who killed 26 church goers in sutherland springs, texas last year from buying a gun. he had been convicted of domestic violence while serving in the air forc