If Congressional Republicans are really intent on getting to the bottom of an ill-conceived sting operation along the border by the Bureau of Alcohol, Tobacco, Firearms and Explosives, they should call President Felipe Calderón of Mexico as an expert witness.

Mr. Calderón has the data showing that the tens of thousands of weapons seized from the Mexican drug cartels in the last four years mostly came from the United States. Three out of five of those guns were battlefield weapons that were outlawed here until the assault weapons ban was allowed to lapse in 2004. To help him stop the bloody mayhem, he is pleading with Washington to re-enact the ban and impose other needed controls.

That is the last thing Representative Darrell Issa, a California Republican, wants to hear. He and Senator Charles Grassley, an Iowa Republican, issued a report last week castigating the A.T.F. for an operation in which federal agents, hoping to track guns to Mexican cartels, monitored but did not stop gun sales to people suspected of obtaining weapons for those criminal groups. Two of the American-sold guns showed up at the scene of a fatal shooting of an American border patrol agent in Arizona last year.

The Justice Department has ordered an investigation, and it must be candid in assessing what happened.