The Mexican government has demanded an "urgent" investigation into how U.S.-grade weapons have ended up in the hands of drug cartels.

"Weapons like this present an extreme danger when they land in the hands of criminals," Kristina Mastropasqua, a spokesperson for the U.S. Bureau of Alcohol, Tobacco, Firearms and Explosives (ATF), told Fox News Digital. "A danger not only to the public, but to the law enforcement agents on both sides of the border as well."

"Operation Southbound is ATF’s primary operational initiative to disrupt the trafficking of firearms from the United States to Mexico," Mastropasqua explained. "Operation Southbound is focused on the four southwest border states, as the majority of the firearms being trafficked to Mexico originate from there, but it is not exclusive to just those states."

"Cross-border firearms trafficking is diffuse, does not only occur on the border, and does not always include dozens of firearms being illegally transported at once; often only a few are trafficked, and they originate in states far from the southwest border," she added. 

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Voice of America reported in June that the number of weapons smuggled into Mexico could top half a million a year, but the total remains uncertain. 

The issue has plagued the U.S. for years. President Obama in 2012 formalized the Border Enforcement Security Task Forces, which focused on tackling cartel activities, including and primarily focusing on arms smuggling, but the issue has not significantly abated, Mexican officials claim.

"The (Mexican) Defense Department has warned the United States about weapons entering Mexico that are for the exclusive use of the U.S. Army," Foreign Minister Alicia Bárcena said.

The Mexican army said it has seized 221 fully automatic machine guns, 56 grenade launchers and a dozen rocket launchers from cartels since late 2018.

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U.S. Ambassador to Mexico Ken Salazar on Monday told reporters that "70% of the weapons that cause violence here in Mexico come from the United States." He stressed that reducing the flow of weapons from the U.S. to Mexico remained a top priority for President Biden, the AFP reported. 

"We are going to look into it, we are committed to working with Sedena (Mexico’s Defense Department) to see what’s going on," Salazar said.

A Reuters report from Dec. 2023 delved into one example of a U.S. weapons factory in Wisconsin that in 2018 allegedly started supplying high-caliber weapons, including sniper rifles, to the Cartel Jalisco Nueva Generacion. 

The report claimed that the cartel exploited "permissive federal and state-level gun control rules to buy some of the most powerful weapons available to American civilians," citing former U.S. ATF agents. 

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Members of a local family in Racine, Wisconsin, with connections to a cousin in Mexico, would buy the guns and ship them to California, where they could then ship the weapons across the border, according to an indictment from Wisconsin’s Eastern District Court. 

Mexican authorities found that same cartel in possession of five rocket launchers during the summer of 2023, with four more launchers confiscated from the rival Sinaloa cartel and three more seized from other cartels. 

"In Mexico, too often, when firearms are diverted to unlawful markets, they are going to arm dangerous drug cartels," Mastropasqua said. "They are getting into the hands of extremely violent organizations that seek to use firearms to further other criminal and illicit activities."

"Many times these cartels are not looking for just any firearm to fuel their criminal enterprises," she continued. "They are seeking a level of weaponry that outguns Mexican law enforcement authorities, including weapons that are used by the American military."

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"ATF’s Mexico Country Office works closely with Mexican authorities to increase the volume and timeliness of firearms tracing through ATF’s eTrace system: That is, tracing a crime gun back to its first known retail purchase," Mastropasqua added. "This capability is incredibly important because so many of the firearms recovered from crimes in Mexico originate in the United States. Between 2017 and 2022, trace submissions from Mexico nearly doubled."

A U.S. federal judge in 2022 dismissed Mexico’s $10 billion lawsuit against seven American gun manufacturers and one distributor, in which officials argued the companies knew the weapons they made would end up sold to traffickers and decided to profit. 

However, on Monday, an appeals court in Boston revived the lawsuit, saying the Protection of Lawful Commerce in Arms Act, which shields gun manufacturers from damages "resulting from the criminal or unlawful misuse" of a firearm, did not apply to such cases that occurred in Mexico. 

The Pentagon referred Fox News Digital to the Mexican government or the U.S. State Department when asked for comment. The State Department did not respond by time of publication. 

The Associated Press and Reuters contributed to this report.