The United States Bureau of Alcohol, Tobacco and Firearms (ATF) arrested 20 suspects on Tuesday in an operation the bureau said disrupted a gun trafficking ring responsible for sending hundreds of weapons to Mexico, according to a U.S. Justice Department press release. Fourteen more suspects remain at large.

ATF Phoenix Special Agent in Charge Bill Newell said the Sinaloa Cartel received the bulk of the weapons, which came from buyers who made multiple, or what’s called “straw” purchases at the behest of middlemen who transported the weapons to Mexico’s border, where they were resold into the black market or directly to cartel associates.

These weapons included multiple AK-47 assault rifles, .50 caliber rifles, and 5.7 mm pistols that carry high-caliber bullets powerful enough to pierce armor and are known as “cop killers,” according to Reuters.

“The investigation is further proof of the relentless efforts by Mexican drug cartels, especially the Sinaloa Cartel, to illegally acquire large quantities of firearms in Arizona and elsewhere for use in the ongoing Mexican drug war,” Newell said.

The bust comes as Hillary Clinton visits Mexico and meets with high-ranking officials, including Mexican President Felipe Calderon, in an effort to persuade Calderon to stay the course in a messy and bloody drug war against mega-cartels whose firepower matches or surpasses the security forces’ and has cost close to 35,000 Mexican lives in the four years Calderon has been president.

“It is meant to intimidate. It is meant to have the public say, ‘Just leave them alone and they won’t bother me.’ But a president cannot do that,” Clinton said in a press conference in reference to the criminals’ penchant for public displays of violence such as beheadings and massacres of rivals.

The Associated Press said Clinton assured Calderon’s government that the United States was stepping up its efforts to slow the trafficking of weapons south. This may include requiring that firearms dealers report the sale of multiple “long guns,” the antiquated terminology that refers to, among other weapons, the assault rifles that arm the powerful drug cartels.

The ATF has asked the White House to change this rule along the border to prevent the type of multiple “long gun” sales that form part of a gun smuggling ring like the one that was disbanded on Tuesday. But any changes are expected to face stiff resistance.

“I think in this Congress … at the rate they are going, they can’t get any firearms legislation,” Dennis K. Burke, the U.S. Attorney for Arizona, told Reuters.

U.S.-based gun-runners are a hot political topic in Mexico. Mexico has stiff laws on owning weapons. Still, an estimated 25,000 guns cross the U.S. border illegally each year. Most of the weapons are purchased in the 6,700 gun stores along the Mexican-U.S. border where stocks of assault rifles abound, and it is legal to purchase multiple “long guns.” It becomes illegal when these purchasers falsify documents at the time of purchase stating why they are purchasing these weapons.

But the Mexican government has also done little to stop arms trafficking once the weapons cross the border. An InSight investigation into arms trafficking in Mexico – to be published in the coming days in conjunction with a larger investigation on arms trafficking with Frontline, the Center for Public Integrity and American University’s Investigative Reporting Workshop – shows that Mexican security personnel are an integral part of the arms smuggling networks in Mexico. These security personnel act as recipients, transporters and salesmen for many of the thousands of weapons the organized crime syndicates use in Mexico.

Steven Dudley is the co-founder and co-director of InSight Crime and a senior research fellow at American University’s Center for Latin American and Latino Studies in Washington, DC. In 2020, Dudley...