A major gun control group is calling on Congress to strengthen background checks by adopting a law it says would have prevented the man who killed nine people last month at a South Carolina church from obtaining a gun.

Everytown for Gun Safety's new ad campaign targets a provision of the law that allows a sale to be issued if a check isn't completed in three business days.

FBI Director James Comey told reporters earlier this month that the accused shooter in the Charleston, S.C., killings was allowed to buy a gun despite a previous drug possession arrest because his background check did not return a definitive answer within that period. 

"It's time for Congress to take action and close the NRA-backed loophole that allows people like the Charleston shooter to obtain a gun without a complete background check," John Feinblatt, president of Everytown for Gun Safety, said in a statement.

"Charleston represents just one of the more than 15,000 sales in the past five years that have gone through without a complete background check. Americans can't sit idly by when we know that all it takes is one gun in the wrong hands to wreak havoc."

Dylann Roof, the man arrested for the Charleston shootings, is currently awaiting trial on murder charges.

Gun rights supporters have panned any attempt to spur new regulations in the wake of the tragedy.