Shannon Watts is the founder of Moms Demand Action for Gun Sense in America , an organization seeking to build support for gun reforms. The opinions expressed in this commentary are hers.

(CNN) Less than a month passed between Charleston and Chattanooga -- two tragedies that shook their communities and the nation at large.

In America, we are constantly tallying up the senseless toll of gun violence. Nine killed in Charleston. Five killed in Chattanooga. A spotlighted fraction of the 88 Americans, on average, killed every day due to gun violence. They don't all make the same number of headlines, but every one of these lives matter.

That's why it's up to all Americans -- political leaders and each of us -- to understand what enabled these tragedies, but also to look forward and ask: What can we do to prevent future shootings such as these -- and the everyday gun violence that doesn't make as much news?

We're still learning the facts about what happened in Chattanooga, however, recent media reports indicate the gunman took advantage of the online gun sale loophole and purchased at least one of his firearms where he knew he could buy a gun with no background check, no questions asked.

And a month has passed since the Charleston shooting, and we're learning a lot about the white supremacist who violently disrupted the sanctity of a Bible study and about how he got his gun. We now know the accused killer bought his gun from a federally licensed dealer with an incomplete background check.

That bears repeating: Dylann Roof was able to buy a gun with an incomplete background check.

Confused? You're not alone. To understand fully how this could have happened, it's important to know the history behind the background check law.

The 1993 Brady background check bill requires gun dealers to run background checks on all gun buyers through the FBI.

But when Congress was debating the bill, the National Rifle Association sponsored -- and successfully pushed through -- an amendment that effectively allows a licensed firearms dealer to sell a gun after three business days, even if the FBI has not yet confirmed that the buyer is legally allowed to have guns.

Most background checks are completed within a few minutes, but in rare cases a check can take longer than three days. In those rare cases, there is something in that potential buyer's background that is uncertain -- something that requires a little more investigation. This doesn't mean that everyone who receives that "to be determined" flag is a prohibited purchaser, but it does mean something in that person's background requires a closer look.

The bottom line is that -- thanks to the gun lobby -- the FBI only gets three days to complete this potentially life-saving process before a gun dealer has the authority -- but not an obligation -- to complete the sale. This arbitrary three-day rule, which the gun lobby fought for, has allowed more than 15,000 guns t o be sold to dangerous people between 2010 and 2014 alone, according to Everytown for Gun Safety

Can you imagine this kind of policy in any other industry? A loan officer would never go ahead and give out money to a bank customer after three days because a credit check was incomplete. A doctor wouldn't tell someone she's cancer-free because her lab results weren't available after three days. And, as a parent, would you want your child's day care worker to be cleared for duty with an incomplete background check?

The NRA was able to strong-arm this amendment into the Brady Bill, making it easier to sell guns and, as a result, American lives are at risk. Congress was wrong: Background checks should be pass or fail. You shouldn't be able to buy a gun with an "incomplete." It's critical that Americans stand up and demand that Congress fix this dangerous loophole so that not one more criminal can buy a gun this way. In fact, U.S. Rep. Jim Clyburn of South Carolina has recently proposed legislation that would do just that.

But we're not just sitting back and waiting for Congress to act. We're also calling on gun retailers to end this dangerous practice and only make gun sales once a background check is fully completed. Despite federal law, retailers have the right and responsibility to set their own policies to help ensure people buying guns from them are not criminals, domestic abusers or dangerously mentally ill.

Many retailers have already had the foresight to close this loophole, such as Walmart, which worked with Mayors Against Illegal Guns to develop a 10-point code to help prevent dangerous people from buying guns, including no sales without a completed background check. But other big-box retailers in the gun business have not put such a policy in place and continue to put the public at risk.

Moms are asking the rest of the nation's largest gun retailers to close this Charleston loophole with our #NoCheckNoSale campaign. First up: Cabela's. In just a few days, we've already gathered more than 100,000 petition signatures, generated nearly 1,000 calls into Cabela's and have taken to social media to spread the word about the retailer's policy. When customers speak up, retailers have an obligation to listen.

After tragedies such as Charleston and Chattanooga, as the details unfold about how something so horrible happened, it's easy to fall into the trap of hypotheticals -- the what if we had done something differently. The truth is we cannot change the past, but we can demand a safer tomorrow.

It is up to everyone -- from political leaders, to gun retailers, to average citizens -- to demand we close every single loophole that allows dangerous people to get their hands on guns. It's common sense, it's gun sense, and it will save lives.