What is more, the sheer volume of licensed dealers  more than 6,600 along the border alone, many of them operating out of their houses  makes policing them a tall order. Currently the A.T.F. has about 200 agents assigned to the task.

Image A gun tied to a killing in Mexico and a dealer in Phoenix. Credit... A.T.F.

Smugglers routinely enlist Americans with clean criminal records to buy two or three rifles at a time, often from different shops, then transport them across the border in cars and trucks, often secreting them in door panels or under the hood, law enforcement officials here say. Some of the smuggled weapons are also bought from private individuals at gun shows, and the law requires no notification of the authorities in those cases.

“We can move against the most outrageous purveyors of arms to Mexico, but the characteristic of the arms trade is it’s a ‘parade of ants’  it’s not any one big dealer, it’s lots of individuals,” said Arizona’s attorney general, Terry Goddard, who is prosecuting Mr. Iknadosian. “That makes it very hard to detect because it’s often below the radar.”

The Mexican government began to clamp down on drug cartels in late 2006, unleashing a war that daily deposits dozens of bodies  often gruesomely tortured  on Mexico’s streets. President Felipe Calderón has characterized the stream of smuggled weapons as one of the most significant threats to security in his country. The Mexican authorities say they seized 20,000 weapons from drug gangs in 2008, the majority bought in the United States.

The authorities in the United States say they do not know how many firearms are transported across the border each year, in part because the federal government does not track gun sales and traces only weapons used in crimes. But A.T.F. officials estimate 90 percent of the weapons recovered in Mexico come from dealers north of the border.

In 2007, the firearms agency traced 2,400 weapons seized in Mexico back to dealers in the United States, and 1,800 of those came from dealers operating in the four states along the border, with Texas first, followed by California, Arizona and New Mexico.