All licensed gun shops and firearm dealer are required to perform background check on buyer to look for any criminal history records. This requirement is mandatory and backed by law.
In order to keep guns out of hands of the fugitives, felons, persons with open arrest warrants, minors, mentally ill and drug addicts, the U.S. Congress introduced the Gun Control Act of 1968.
This Act, however, had loopholes that allowed such people to get their hands on guns. Gun control act covered licensed firearm dealers but there are many sellers that sell guns casually.
These unlicensed gun sellers allowed people to purchase guns without going through any criminal history background checks. These gun buyers also included criminals and people with disruptive intents.
No wonder a large portion of gun violence have seem involvement of guns that were purchased without performing any criminal history background checks on buyers.