WASHINGTON -- More than a dozen Democratic senators, including Sen. Elizabeth Warren (D-Mass.), announced their support on Wednesday for new legislation to close a gun purchase loophole that the FBI says allowed accused Charleston shooter Dylann Roof to buy one of the weapons used in a deadly June attack in a South Carolina church.

The legislation, introduced by Sens. Richard Blumenthal (D-Conn.), Chris Murphy (D-Conn.) and Dianne Feinstein (D-Calif.), would require that a background check be completed before any gun sale proceeds. The FBI currently allows a federally licensed firearms dealer to sell a gun within 72 business hours , even if a background check from the National Instant Criminal Background Check System has not concluded, though some states and retailers still prohibit the sale from being completed. In 2014, the NICS completed 91 percent of background checks immediately.

Citing FBI data that showed 15,700 prohibited sales have gone through because of the loophole over the last five years, Blumenthal said waiting more than 72 hours for a background check was a "minor inconvenience."

No check, no sale must be the rule -- enacted as law -- to close a gaping loophole spreading the epidemic of gun violence, he said in a statement. "Despite the distortion and dishonesty of single-issue groups like the NRA, the vast majority of Americans, including gun owners, believe in background checks and want the system to work well.

When Roof tried to purchase a .45-caliber handgun in April, an examiner conducting a background check had trouble locating the correct records to show that Roof should be blocked from purchasing the weapon, according to The New York Times . When the examiner couldn't make a determination within 72 hours, Roof was allowed to purchase the gun.

The shooter in Charleston would have failed his background check if the law had allowed it to be completed. This commonsense proposal will ensure all background checks are completed before sales can move forward," Feinstein said in a statement.

Jennifer Baker, an NRA spokesperson, said the real issue is bureaucracy, not the amount of time allowed for a background check. The tragic shooting in Charleston, which this bill's proponents say is the driving force behind the legislation, occurred 64 days after a federal background check was completed," she said.

"The problem in this instance was a lack of communication among federal and state agencies, not too little time to complete a background check. Sens. Blumenthal, Murphy and Feinstein should focus on actually solving problems, instead of using a tragedy to attack the constitutional rights of law-abiding citizens."

The measure, called the Background Check Completion Act, is being co-sponsored by Democratic Sens. Dick Durbin (Ill.), Barbara Boxer (Calif.), Jack Reed (R.I.), Robert Menendez (N.J.), Sheldon Whitehouse (R.I.), Kirsten Gillibrand (N.Y.), Mazie Hirono (Hawaii), Tim Kaine (Va.) and Ben Cardin (Md.), in addition to Warren.

At a press conference on Wednesday morning, Warren said the bill stood as a test of the Senate's values. "A bill that is this straightforward is a test for the United States Senate. It asks the fundamental question, 'Who do you work for?'" she said . "So long as we don't act, we make it clear that this Congress works for the NRA."

Rep. James Clyburn (D-S.C.) introduced a similar bill in the House in July that's currently sitting in committee. It will be an uphill battle to get the legislation passed, as Congress has largely failed to pass meaningful gun reform in the wake of mass shootings.

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