U.S. federal investigators say it´s a simple matter of supply and demand.

Warring Mexican gangs need weapons and are reaching into the bountiful and legal supply just across the border to build their arsenals.

Because of rigid firearms restrictions in their country, authorities say, Mexican criminals are increasingly dispatching operatives to sporting goods stores, gun shows and flea markets in Texas and other states to load up on assault weapons, pistols, shotguns and ammunition.

The firearms are then smuggled into Mexico to become implements in the country´s drug wars and other criminal activity.

"It´s quite widespread," said J.J. Ballesteros, chief of the Corpus Christi (Texas) office of the Bureau of Alcohol, Tobacco, Firearms and Explosives (ATF) and one of the agency´s leading experts on firearms trafficking in Mexico.

SOURCE STATE

With thousands of firearms dealers and a 1,240-mile-long border with Mexico, Texas is an easily accessible "source state" for cross-border gun traffic, investigators say.

The transactions typically involve "straw purchasers" - U.S. residents with clean records - who get widely varying fees to buy weapons for Mexican customers.

ATF agents in Mexico trace the origins of weapons recovered from crime scenes and suspects. Between 5,000 and 7,000 firearms are traced each year, Ballesteros said, and most come from the United States.

He said he believes the confiscated weapons represent "only a drop in the bucket" in the flow of arms from the United States.

Mexican officials have appealed for help from the U.S. government to help control firearms trafficking.

ATF Director Carl J. Truscott met with his Mexican counterparts late last year in what U.S. officials described as a "shoulder-to-shoulder" assault against the flow of guns.

A key element of the assault is the 9-month-old Operation Black Jack, led by U.S. Immigration and Customs Enforcement (ICE) to combat the surge of violence along the border. The operation includes the ATF and other U.S. agencies as well as Mexican law enforcement.

In October, members of the Black Jack team looking for a slaying suspect raided a home in Laredo and found 10 automatic weapons being readied for shipment across the border. An illegal immigrant arrested in the raid was guarding the guns for a cell controlled by Mexican crime boss Joaquín "El Chapo" Guzmán, a leader of one of the feuding drug cartels, ICE officials said.

Investigators say their cases often unfold after customers are seen leaving retail stores with what appears to be an unusual amount of weaponry or ammunition. Store employees also tip officials to oversized purchases.

Last year, ICE agents in Brownsville arrested a married couple trying to transport 17,650 rounds of ammunition into Mexico after they were seen leaving a Wal-Mart with a large purchase. The couple stockpiled much of the ammunition in a bus station locker, investigators said.

José Luis González, a Laredo gun-store owner and former assistant high school band director, began serving a 37-month sentence in May for arranging the fraudulent purchases of 86 firearms.

Witnesses told ATF agents that González paid them US$50 to US$100 to falsify firearms-transaction forms. Some of the guns, witnesses said, were intended for Mexican drug dealers.

"It´s a very big problem on the border, especially here in Laredo," said Homero Arce, a gunsmith at Brush Country Guns in Laredo. "I´m no longer selling more than two handguns. A lot of people are doing that around here."

Jorge Montiel, manager of Brownsville´s Wal-Mart Super Center, said his employees work with federal agents and alert authorities to dubious activities. "We let them know of any suspicious purchases," Montiel said by phone.

SURGING DEMAND

U.S. border towns offer the easiest access to customers in Mexico, but an increasing number of weapons are being purchased in Dallas-Fort Worth and Houston for shipment south of the border, says Al Peña, agent in charge of ICE´s San Antonio office.

"Most of these people are opportunists looking for what they believe to be easy money," Peña said. "There´s no turning back once they´ve involved themselves."

In addition to firearms from the United States, Peña says, investigators have recovered weapons traced to the Mexican military. A smaller amount comes through the international black market from countries in the former Soviet Bloc.

The surging demand in Mexico is fueled largely by a gang war to control one of the nation´s richest drug-smuggling corridors, in the Nuevo Laredo area, across from Laredo. Authorities say other potential customers include garden-variety criminals, kidnapping rings and revolutionary groups operating deep in the country´s interior.

Mexico has one of the most restrictive gun control laws in the hemisphere, permitting ownership of a few types of guns, mostly small caliber. The nation of 103 million has fewer than 2,500 registered gun owners, according to a recent report in the Los Angeles Times, and the wait for a license can often be more than a year.

By contrast, a smorgasbord of firepower is available in the United States, where a ban on assault-style weapons expired in September 2004. Among the most popular firearms for Mexican purchase, federal investigators say, are semi-automatic ARs, AKs and Tec-9 pistols.

Customers of federally licensed gun dealers are required to show identification and fill out an application. An instant background check determines whether the customer has a felony conviction or other legal blemishes that would prevent a purchase.

Private owners, who often sell part of their collection at gun shows, are not required to conduct a background check. In Texas, people can buy guns and ammunition in unlimited quantities, according to the ATF, but excessive purchases might cause suspicion.

"You can literally outfit an army out of a gun store, and it´s being done," said Ballesteros, of the ATF.

INCREASINGLY BRAZEN

The criminal cartels often arrange gun traffic through the same organizational structure used for smuggling drugs, Ballesteros said. A criminal boss, he said, might place a weapons order with a high-ranking subordinate, who will arrange for a middleman in the United States to recruit a group of purchasers, often friends or relatives who have never had a brush with the law.

Payments vary. A group of straw purchasers in Douglas, Arizona, got US$400 for each AK purchase. Other times, the commission is a percentage of the cost of the gun. Purchasers are often paid in drugs instead of money.

The weapons are smuggled into Mexico in a variety of ways, often in hidden compartments in vehicles or aboard inner-tube floats across the Rio Grande. Smugglers also attempt to conceal them while walking across international bridges.

"The weapons are making their way southbound, and who knows how many are getting by without getting stopped?" Webb County Sheriff Rick Flores of Laredo said.

"I don´t think the violence is going to lessen. I think it´s going to get worse."