Listings for semi-automatic rifles on the popular online gun website Armslist.com, September 27, 2013.

Last October, while most of the country was obsessed with the presidential election only a week away, Radcliffe Haughton walked into a Wisconsin spa with an entirely different obsession. Earlier in the month, Zina Daniel, his estranged wife, obtained a restraining order against him after a long history of domestic abuse and stalking. She worked at the spa, and he had visited before—seventeen days prior, he slashed her car’s tires in the parking lot outside.

But now he came with more than a knife: Haughton had a .40 caliber semi-automatic Glock handgun with him. Immediately upon entering the store, he shot Zina dead—and then murdered one of her co-workers, and then another. He continued shooting, and four other people were wounded before he finally turned the gun on himself.

The judge’s restraining order specifically forbade Haughton from buying a gun, and had he gone to any licensed firearms dealer, he presumably would have been denied. Instead, Haugton went to the increasingly popular Armslist.com, where tens of thousands of guns are up for sale, and where the vast majority of the sellers are private citizens. Within days he had the Glock.

Armslist.com is only a part of the online gun marketplace, but it is a big part—there were almost 100,000 listings on the site in August. The site, and many others like it, went from being virtually unknown to hugely popular in a matter of months, as other mainstream online retail markets, like Craigslist, cracked down on gun listings:

Courtesy Mayors Against Illegal Guns

A new study by Mayors Against Illegal Guns has revealed that every thirtieth gun sold on Armslist.com was sold to someone who would have failed a background check. Of 607 would-be buyers in the sample, which stretched from February to May of this year, 3.3 percent were sold to someone who committed a crime that prohibited firearm possession under federal law.

That ratio is many magnitudes higher than the one for licensed gun dealers—based on the rate of background check denials, only 0.87 percent of would-be buyers at licensed dealers are prohibited purchasers. The report gives a troubling analogy for the Armslist.com marketplace: if one in every thirty passengers on a Boeing 747 were on the federal terror watch list, there would be twenty-two suspected terrorists on board.

And in fact, the ratio is likely much higher. Mayors Against Illegal Guns had a relatively simple methodology—the group examined 13,000 “want-to-buy” ads where the buyer entered a name and contact information, and then compared that information to publicly available criminal records.

So the one-in-thirty result is almost definitely an undercount, because it doesn’t include people who are precluded from gun ownership for other reasons: serious mental illness, immigration status, a history of drug abuse or other non-criminal criteria.

But more notably, only 5 percent of the people who place “want-to-buy” ads on Armslist.com disclose their name and contact information. So one could fairly assume that among the people who choose anonymity, the ratio of prohibited buyers is higher. The analysis also doesn’t, and couldn’t, take into account the people who simply respond to the sellers’ ads directly—another likely approach if someone knows they can’t buy a gun legally.

And again, Armslist.com is only one part of the huge online gun marketplace; Mayors Against Illegal Guns is just scratching the surface here. But other investigations have borne similar results. In 2011, investigators working for New York City called 125 sellers in fourteen states advertising on ten different websites, including Armslist.com. The investigators plainly said they were unlikely to pass a background check—and 62 percent of the sellers agreed to transfer a weapon anyway. On Armslist.com the rate was 54 percent.

These are exactly the kinds of sales the Manchin-Toomey gun legislation that failed in the Senate earlier this year was designed to stop. Under that bill—which 90 percent of the public supported—a background check would be applied almost universally, including, crucially, for online sales.

“Loopholes in our federal gun laws have taken a devastating toll on communities and neighborhoods across America. Yet in April, a minority of US senators blocked the common-sense legislation that would have closed these deadly loopholes,” said Boston Mayor Thomas Menino when the study was released. (He is a co-chair of Mayors Against Illegal Guns.) “It’s time for our leaders in Washington to put public safety first and pass sensible gun laws that will help save lives.”

The common refrain from pro-gun activists is that criminals will avoid background checks anyway—which, as this study shows, is true. But maybe we shouldn’t make it so easy for them.

Marie Myung-Ok Lee on breaking the cycle of anger.