In the escalating drug war south of the border, Mexican cartels supply the drugs, but the guns largely come from the United States.
"Firearms trafficking to Mexico is a huge problem," says Phoenix ATF agent William Newell. "Drugs go north, guns come south."
George Iknadosian is accused of being a top gun-supplier. When government agents raided his Phoenix gun shop last May, they found hundreds of weapons allegedly destined for Mexico. He's now on trial, accused of knowingly selling more than 700 guns to so-called straw buyers - U.S. citizens who buy the guns legally and then turn them over to a trafficker.
"They get $100 for their trouble, and the trafficker will take the gun down to Mexico and sell it for exponentially more than they pay here," says ATF agent Peter Forcelli.
As many as 2,000 firearms are believed to cross the border into Mexico every day. And they are often assault weapons, Tracy reports.
The ATF says that 7,700 guns found in Mexico last year were traced to sellers in the United States.
"All the killing they are doing here are killing with guns that are selling in the United States," said Tijuana Mayor Jorge Ramos.
Mexican law makes it nearly impossible to buy guns there legally. But less restrictive laws in the U.S. keep the firearms flowing over the border. Court papers in the Iknadosian case claim U.S. border states provide three-quarters of black market firearms to Mexico.
And with more than 2,000 people killed so far this year in drug-related violence in Mexico, cutting off the gun supply is now a top concern on this side of the border.