Last week, in his hometown of Chicago, President Barack Obama appealed to a conference of law enforcement officials to help him push for stricter federal gun laws. Anticipating what his critics might say, Obama addressed an apparent paradox: Chicago has restrictive gun regulations, yet continues to record thousands of shootings per year. To turn that into a case against gun reform, he continued, is to misunderstand how the flow of illegal guns works. “The problem with that argument, as the Chicago Police Department will tell you, is that 60 percent of guns recovered in crimes come from out of state,” Obama said. “You’ve just got to hop across the border.”

Data from the Bureau of Alcohol, Tobacco, Firearms and Explosives (ATF) backs up the president’s point. The agency cannot trace every gun taken in by law enforcement. But between 2010 and 2014, it was able to source between 40 and 60 percent of the firearms recovered in Illinois, the vast majority of which were crime guns. Statewide, most of those weapons came from elsewhere in Illinois, a pattern seen in other states. But thousands found their way into Illinois — and often, Chicago — from parts of the country with weaker gun laws.

Here’s what that looks like, focusing just on the 15 states that send the most crime guns to Illinois:

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While the Windy City outlaws gun stores, straw purchasers can pick up firearms in neighboring suburbs that have track records of failing to police the gun sellers within their borders. Across the state line in Indiana, gun laws are loose enough to earn the state 17th place on Guns and Ammo‘s list of the best states for gun owners (Illinois ranks 43rd).

Not coincidentally, as the visualization above shows, in 2010, 2011, and 2014, the annual count of Illinois crime guns originating in Indiana topped 1,o00 guns per year. (In 2012 and 2013, there was a big dip in Illinois crime guns coming from Indiana, though the ATF isn’t sure why.) Mississippi was next in line, trafficking about a third as many guns into the state. At least four others exported more than 500 guns to Illinois during 2010–14. Five more states sent more than 400 each.

The top 15 source states for Illinois crime guns, 2010–2014:

  • Indiana 3,269
  • Mississippi 1,002
  • Wisconsin 898
  • Missouri 780
  • Kentucky 503
  • Tennessee 459
  • Georgia 453
  • Ohio 448
  • Texas 444
  • Florida 439
  • Michigan 365
  • Iowa 344
  • Alabama 315
  • Arkansas 280
  • Other 407*

*Several states, such as California, were in the top 15 some years but not others; we’ve totaled those states here.

Across the country, guns make their way across state lines, and into crime scenes, in similar fashion. In Chicago, it’s why police can seize an illegal gun every 75 minutes but fail to stop the tide. And nationally, it’s why the chief of the ATF’s violent crime and intelligence division has compared trafficked guns to cockroaches in an apartment complex. If you aggressively treat the problem in one place, while leaving it unchecked elsewhere, the infestations will continue.

With additional reporting by Olivia Li.