One chart that tells the story: 70 percent of guns seized in Mexico and traced by the US Bureau of Alcohol, Tobacco, Firearms, and Explosives (ATF) from 2009 to 2014 came from the US:
As the GAO notes, these are only guns that are seized in Mexico and traced by the ATF, not all guns in the country. It is possible, perhaps likely, that the guns the ATF is tasked with tracing are predominantly from America. Still, 73,684 firearms is a lot of guns, so US weapons are definitely contributing to the tremendous levels of violence in Mexico.
The GAO also reported:
ATF data also show that these firearms were most often purchased in Southwest border states and that about half of them were long guns (rifles and shotguns). According to Mexican government officials, high caliber rifles are the preferred weapon used by drug trafficking organizations. According to ATF data, most were purchased legally in gun shops and at gun shows in the United States, and then trafficked illegally to Mexico.
The GAO noted that in addition to traditional trafficking, many guns are trafficked as different weapon parts that are put together once they're in Mexico. These guns are much more difficult — sometimes impossible — to track.
More broadly, the report shows the wider consequences of America's relaxed gun laws. The empirical research is clear that the abundant access to guns in the US leads to more gun deaths in the US. But it also apparently leads to more gun deaths in Mexico.
The lesson: Strict gun laws can't totally work if a neighboring government has lax laws
Mexico is one of the few countries that, like the US, guarantees the right to bear arms in its constitution. Still, Mexico maintains some fairly strict gun laws: All guns must be registered through the federal government, carrying a gun requires a license, sales are legally limited to one store in Mexico City, and carrying licenses can be taken away at the federal government's discretion.
The US, by comparison, doesn't have these standards, they can be bypassed, or they vary from state to state. (For more on US gun laws, and how they compare with those of other developed countries, read my explainer.)
So one way for Mexicans to get around their country's strict gun laws is to simply walk across the border. To visualize this problem, look at this map from MetricMaps, which shows just how many gun dealers are on the US side of the US–Mexico border, a sharp contrast to just one legal gun store in Mexico:
This is a problem we see in the US too: No matter how strict your gun laws are, it's going to be hard to stop the flow of guns if someone can simply go to a neighboring state or country and buy a gun easily. Christopher Ingraham at Wonkblog, for example, documented that many of the guns used in crimes in the US come from states with lax gun laws.
So while the research clearly shows that more guns mean more gun deaths — and less access to guns can reduce gun deaths — ultimately any impact gun control laws have will be diminished if a neighboring government doesn't enforce equal or stricter laws.
For Mexico, this has abhorrent consequences: The country's war on drugs has led to so much violence that men's life expectancy fell after its decades-long trend upward. But it seems gun policy, not just drug policy, played a role in worse levels of violence too.