Plus you got to remember the background checks are done by humans and errors or the good ole boy system can occur. Years ago if you were arrested for a felony and not fingerprinted, or the fingerprints rejected or lost it was not entered into national NCIC/FBI database.
GC ....the agency you did a background check hired a person with a felony.
Plus he was probably issued a gun license. Something that a lot of states would have rejected.