Mayors Against Illegal Guns (MAIG), a national gun violence prevention organization co-founded by NYC mayor Michael Bloomberg, was featured in the Philadelphia Metro today highlighting the results of a study they conducted revealing the prevalence of illegal gun purchases via the internet (read the full story here).

The study focused on ArmsList.com, which Bloomberg calls “the Craigslist for national gun sales.” On this website, private gun owners and dealers wishing to sell firearms can post ads that anyone wishing to purchase a gun can search for in order to make contact directly with the seller. The study’s report claims that in just one year, a single website such as this one can facilitate the purchase of over 25,000 guns by convicted felons and others not legally allowed to purchase or handle firearms.

The problem with these websites is that “federal law doesn’t require private sellers—online or otherwise—to conduct background checks,” says Bloomberg. In Pennsylvania, private sellers of long guns are not required to conduct background checks, enabling the acquisition of many illegal guns throughout the state.

A man, days after being slapped with a restraining order which made him ineligible to purchase guns, bought a .40 caliber Glock on ArmsList.com and used it to kill his wife, two of her co-workers, and himself last year in Wisconsin.

Bloomberg goes on to say “mass shootings—like the one at the Washington Navy Yard—will continue until we get serious about this issue, citing gridlock by Congress.”

Philadelphia mayor Michael Nutter, a MAIG member, cites universal background checks as one of the most effective weapons lawmakers can use to keep guns out of the hands of people who should not have them. Nutter compares the lack of background check requirements for private sellers to “setting up two different lines at the airport – one where you have to go through security and one where you don’t” and asks “which one do you think the criminals are going to choose?”