It’s not difficult to grasp how a man with a history of gun arrests and mental instability obtained the necessary firepower to commit a massacre at the Washington Navy Yard earlier this week. Buying a gun, or even an arsenal, is exceptionally easy in the U.S.

Even so, there are some people who both fail to meet the minimal requirements for purchasing a gun from a licensed dealer and whose relevant personal details are known to the Federal Bureau of Investigation. Since 1998, the FBI’s National Instant Criminal Background Check System has stopped more than 2 million attempted gun purchases by felons, domestic abusers, drug addicts, the mentally ill and other prohibited persons.

Those blocked from making legal purchases from gun dealers, however, have an alternative marketplace -- online. As a report released this week makes painfully clear, that marketplace is growing in both size and danger.

Mayors Against Illegal Guns, cofounded by New York City Mayor and Bloomberg LP founder Michael Bloomberg, found 83,000 ads for guns on Armslist.com, a site devoted to firearm sales. The number of criminals seeking guns on the site -- federal law requires no background checks or questions of any kind for in-state, nondealer transactions -- would be astounding if it weren’t so predictable.

So when Radcliffe Haughton was prohibited from buying firearms, he naturally turned to Armslist, where he found a willing seller while avoiding a background check or any other kind of scrutiny. Haughton bought a .40-caliber semi-automatic Glock handgun. The next day, he drove to his estranged wife’s workplace and murdered her along with two co-workers. Four others were injured before Haughton turned the gun on himself.

This is the “gun-show loophole” in action. Armslist is precisely the kind of lawless marketplace that the National Rifle Association champions and that the U.S. Senate this year mobilized to protect by scuttling background-check legislation. With the growth of online markets, there’s never a need to wait for a gun show; tens of thousands of guns are on sale every hour of every day.

In an effort to gauge the extent of criminal activity online, Mayors Against Illegal Guns analyzed the contact information supplied by prospective buyers in the “want-to-buy” ads posted on Armslist. According to the study, 1 of every 30 gun seekers in the want-to-buy section had a history that would prohibit them from legally possessing a gun.

“Alarming as this snapshot is,” states the report, “it badly understates the true scope of the problem. Only 5 percent of the postings on Armslist are want-to-buy ads: the vast majority of buyers -- prohibited and otherwise -- respond to ‘for-sale’ ads, and therefore remain completely anonymous.”

There are rumblings in Washington that background-check legislation will be resuscitated on the Senate floor. If so, it should be a meaningful exercise. So perhaps it’s best if Americans first ask themselves a simple question: How easy should it be for criminals and the mentally ill to buy guns? As one massacre after another confirms, such people enjoy virtually unfettered access to guns right now. The gun lobby will continue to work to preserve that status quo. What are the rest of us willing to do to change it?

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