The so-called “default to proceed” rule under the Brady Handgun Violence Prevention Act allows firearm dealers to sell a weapon to an individual if they have not been notified by the FBI of a buyer’s criminal background within three business days.

If the background check has not been completed within three days, the dealer has discretion to sell the firearm.

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The rule allowed Dylann Roof, the alleged killer of nine people at a predominantly black church in Charleston, S.C., to purchase a gun even though he was arrested on a felony drug charge and admitted to possessing drugs.

"We shouldn't give known criminals the benefit of the doubt when it comes to guns. If law enforcement needs more than three days to ensure they're not giving weapons to dangerous people, Washington must allow them the time to do their jobs,” Blumenthal and Murphy said in a joint statement.

“If we refuse to act, we're just biding time until this happens again."

The criminal background check on Roof was not initiated until two days after Roof first attempted to purchase the gun and did not discover his admission of drug possession until five days later.

FBI Director James Comey told reporters Friday that Roof “should not have been allowed to purchase the gun he allegedly used that evening,” referring to the deadly June 17 attack.

Republicans, however, blame the failure to bar Roof from obtaining a gun on “bureaucratic bungling” and say additional regulation is not necessary.

“It’s disastrous that this bureaucratic mistake prevented existing laws from working and blocking an illegal gun sale. The facts undercut attempts to use the tragedy to enact unnecessary gun laws,” Senate Judiciary Committee Chairman Chuck Grassley (R-Iowa) said in a statement.

An FBI examiner faxed a request for information on Roof’s arrest to the wrong county’s sheriff’s office.

Comey has announced a 30-day investigation into the matter.