The Mexican Senate on Thursday called a hearing over reports that U.S. agents allowed guns to be smuggled into Mexico as part of investigations into drug traffickers.
The Senate voted to summon U.S. Ambassador Arturo Sarukhan to discuss the issue, though it set no date. The lawmakers also asked Foreign Relations Secretary Patricia Espinoza to demand information from the U.S. State Department.
CBS News and the nonprofit Center for Public Integrity reported that agents of the U.S. Bureau of Alcohol, Tobacco, Firearms and Explosives investigating gunrunning by cartels allowed hundreds of guns purchased in the U.S. to go into Mexico.
"Carrying out this type of operation in our country is unacceptable. It violates trust and also undermines national sovereignty," said Sen. Luis Alberto Villarreal, a member of President Felipe Calderon's National Action Party.
U.S. Attorney General Eric Holder has asked the Justice Department inspector general to look into ATF investigative strategies and determine whether further investigation is needed.
"It is true, there have been concerns. I take those allegations very seriously and that is why I asked the IG to report on it," Holder told a U.S. Senate appropriations subcommittee Thursday. "Letting guns 'walk' is not something that is acceptable. Guns are different than drugs or money when we are trying to follow their trail."
Gun smuggling has long been a sore spot in relations between the two countries. Mexico frequently demands that the U.S. do more to stop the flow of weapons south.
The debate sharpened last month when a U.S. Immigration and Customs Enforcement agent was killed on a northern Mexican highway with a gun that was purchased in a town outside Fort Worth, Texas.
The concerns about the ATF is the latest source of friction between the U.S. and Mexico, which for years have been touting their increased cooperation in the fight against drug traffickers.
Calderon has been furious about leaked U.S. diplomatic cables describing inadequate coordination among Mexican federal agencies assigned to battle drug gangs.
The latest document, obtained from Wikileaks by the Mexican newspaper La Jornada and published Wednesday, indicated that U.S. officials were baffled by the 2009 appointment of Mexican Attorney General Arturo Chavez Chavez, calling it "totally unexpected and politically inexplicable."
Chavez Chavez came to office under a cloud of controversy over botched prosecutions in the murders of women in the northern Mexico border city of Ciudad Juarez in the 1990s, when he was the top state prosecutor.