The Bureau of Alcohol, Tobacco, Firearms and Explosives traced more than 211,000 guns back to their original point of purchase on behalf of law enforcement agencies in 2016. Of those, roughly 71 percent were originally sold in the state they were found, while the rest were from out of state. This interactive map shows the pattern of how guns move from state to state.

Why it matters: It's often impossible to establish the chain of ownership from owner to original point of sale. Firearms legally change hands without a paper trail through private purchases, including those made at gun shows. Additional restrictions on how records can be stored can make the entire chain even harder to trace.

Most ATF traces are run at the request of law enforcement agencies seeking information on guns recovered at crime scenes or confiscated during traffic stops. Not all recovered firearms are traced, but those that are can tell us a lot about how guns move between states. Here are some key takeaways from the data: Roughly 71 percent of guns traced by the ATF in 2016 originate in the same state in which they were confiscated. Washington, D.C. had the highest ratio of out-of-state traces at 96 percent. New Jersey and New York were the next highest, at 79 percent and 73 percent. States with stricter gun laws tend to have more traces originating in neighboring states. Illinois requires background checks for private sales and a 72-hour waiting periods for handgun purchases. Neighboring Indiana does not. Thirty percent of out-of-state guns in Illinois originated from Indiana. Go deeper: Gun Laws Stop At State Lines, But Guns Don't