A woman examines a pistol at a 2016 gun show in Fort Worth where thousands of weapons were for sale. (Spencer Platt/Getty Images)

The school shooting in Parkland, Fla., that claimed the lives of 17 children and educators has sparked a renewed interest in gun-control legislation in statehouses across the country.

Just this week, Florida legislators approved the state's first major gun-control package in more than 20 years, defying opposition from gun-rights groups like the National Rifle Association. Gun-control opponents like the NRA often argue that most gun-control laws aren't effective because criminals “don't follow gun laws.” They point to high rates of gun violence in places with strict gun laws, most famously Chicago, as evidence for this claim.

But that argument overlooks the simple fact that firearms travel easily over state and municipal lines. A Washington Post analysis of five years of gun-trace data from the Bureau of Alcohol, Tobacco, Firearms and Explosives bears this out: From 2012 to 2016, more than a quarter-million firearms were purchased in one state and used in a crime in another. Those guns typically flowed from states with lax gun laws to those with more strict regulations. And in a number of states, including Illinois, California and New York, at least half of the guns used to commit crimes were purchased out of state.

When a firearm is recovered from a crime scene, law enforcement can request a trace from ATF to identify where the gun was sold and who bought it. Each year ATF releases summary reports detailing, at the state level, where crime guns were recovered and which states they originated from. It defines these “crime guns” as “any firearm that is illegally possessed, used in a crime, or suspected to have been used in a crime.”

Nationwide, most crime guns are purchased in the same state they're recovered from. In 2016, for instance, of the 211,000 crime guns for which ATF was able to identify a purchaser and a dealer, more than 150,000 were bought in the same state in which the crime was committed.

But that's not the case in every state. In five states and the District of Columbia, out-of-state gun purchases accounted for at least half of all recovered crime guns from 2012 to 2016. In New Jersey, 79 percent of recovered guns were purchased out of state. The rate was 70 percent for New York, 60 percent for Massachusetts, 53 percent for Hawaii and 50 percent for Illinois.

Out-of-state crime guns tend to be purchased in states adjacent to where they're recovered, particularly if those neighboring states have looser gun laws. To see where these interstate crime guns are disproportionately likely to be purchased, we can look at the rate of guns trafficked out of state per 100,000 people. This is preferable to using raw numbers of trafficked guns because it corrects for population differences between states.

There's a wide variation in rates of gun trafficking between states. Places like Mississippi, West Virginia and Alaska, which have some of the most permissive gun laws in the country, exported more than 200 crime guns for every 100,000 residents between 2012 and 2016.

Compare those figures to ones from a state like Connecticut, which exported 13 crime guns per 100,000 residents, or New York, which exported 15 crime guns per 100,000. In short, a gun bought in a place like Mississippi or West Virginia is more than 10 times as likely to be used in an out-of-state crime as a gun bought in Connecticut or New York.

But we're still only looking at half the picture. To get a more complete sense of the interstate flow in crime guns, we can compare the rates of gun imports and gun exports in each state.

In the map above, the states in red export more crime guns per capita than they import. On net they are sources of crime guns. States in blue import more than they export. They're net magnets.

There's a substantial correlation between the strength of a state's gun laws and whether it's a net importer or exporter of crime guns. States like California, New York, Illinois and Maryland have some of the toughest gun laws in the nation. They're also magnets for crime guns purchased out of state.

From 2012 to 2016, for instance, 2,930 crime guns found in other states were traced back to New York, a rate of about 15 crime guns for every 100,000 New Yorkers. But over the same period, 17,229 crime guns were recovered in New York and ultimately traced to purchases in other states, a rate of 87 crime guns for every 100,000 New Yorkers. On net, the state imported more than 14,000 more crime guns than it exported, for a net rate of 75 imports per every 100,000 New Yorkers.

The implication is that would-be criminals in New York (and states like it) are traveling across state lines to find more favorable gun purchasing environments. Or, it could be that gun traffickers are buying guns out of state and bringing them to places like New York to sell. Most likely a combination of both factors is at work.

Research has shown that the overwhelming majority of guns used in crime were initially purchased lawfully. But through loss, theft and other channels, those guns make their way into the hands of people who use them for illegal purposes.

Researchers have found that a number of state-level policies are correlated with reductions in the diversion of guns to criminals: state licensing requirements for dealers, regular inspections of dealers, mandatory reporting requirements for loss or theft, background checks on private sales, and permit requirements to buy firearms, to name a few.

But the maps and figures above underscore the shortcomings of writing gun policy at the state level. Strict gun laws in one state can only do so much if there is a more permissive purchasing environment just across the state line. Certain policies, such as universal background checks or permitting requirements, would be much more effective if they were implemented nationwide.

Despite overwhelming public support for policies like these, Congress hasn't enacted any new gun-control legislation since 2007.