McALLEN, Texas — In the graceless Mexican border town of Reynosa, just a few miles over the Rio Grande from here, federal police were stunned by what they found in a Gulf Cartel safe house last November.

Before them lay more than 500,000 rounds of every type of ammunition, sticks of dynamite, 165 hand grenades, tear gas launchers with plenty of shells, handguns and 540 assault-style rifles. It was billed as the largest seized weapons cache in Mexican history. (Click here for a guide to Mexico's leading drug cartels.)

This week, GlobalPost learned the U.S. Bureau of Alcohol, Tobacco, Firearms and Explosives has traced the majority of those assault-style rifles, by serial number, straight to licensed Texas  dealers.

The finding underscores the reasons why U.S. Homeland Security Department Secretary Janet Napolitano will end her trip to Mexico this week with a stop in Laredo, Texas. She will visit border security systems — through which many of these weapons were no doubt smuggled south after purchase.

Texas retailers — particularly those that inhabit the southern border regions where McAllen and Laredo hug the Rio Grande — lead other U.S. retailers as the origin of powerful weapons and ammunition flowing into cartel depots in Mexico.

"We know that Texas is a leading source state for firearms going into Mexico," said Robert Elder, second in command of the ATF's Houston field office, which covers hundreds of miles of border. "This further confirms that."

Napolitano and U.S. Attorney General Eric Holder will arrive in south Texas today, April 3, after attending a firearms smuggling conference in Cuernavaca, Mexico, and after discussing Mexican President Felipe Calderon's longstanding complaints about the smuggling of American guns into Mexico, where they arm the government's mortal enemy: the drug syndicates.

ATF statistics show that in 2007, the last full year for which figures were available, Texas sellers were the source of 1,131 traced guns found discarded at Mexico shooting scenes or confiscated from cartel gangsters. That's more than twice the number of California sellers — which was the runner-up to Texas — and more than the combined total of 13 other top states.

Tracing results for 2008 won't be available until next month. But they are expected to show a dramatic increase in weapons traced to Texas shops. The prevalance is due to several factors:  culture, a dearth of sales regulations, and a proliferation of sporting goods retailers close to a long, porous border.

U.S. and Mexican law enforcement officials also say Texas hosts the nation's busiest corridor for ammunition smuggling into Mexico.

Mexico is demanding that that the Americans tighten gun regulations, a politically dicey proposition considered unlikely to advance quickly — if at all. So instead, the Obama administration and Congress recently earmarked millions of dollars to beef up the ATF's ability to sniff out the trails of gun buyers and their trafficking partners working Texas towns and cities.

The vast majority of these agents — some 170 — will be sent to Texas to join "Operation Gunrunner." The investigative program seeks to follow the trail of traced weapons from retailer to buyer to smuggler.

The Reynosa seizure alone will provide the agents with plenty of new leads. Of the 383 guns the ATF was able to trace, more than 300 came from Texas retailers, GlobalPost was told. The other weapons were traced to seven other states, including Illinois, Florida, North Carolina, Virginia and Michigan.

ATF officials declined to provide the names of specific retailers, on the grounds that public knowledge could compromise impending investigations.

Attacks averted?

The Reynosa weapons seizure is noteworthy for more than its size.

The seizure took place just in time to avert what some knowledgable officials said could have been the Gulf Cartel's first open attacks on U.S. officers and agents.

The weapons depot had been under the control of a notorious cartel lieutenant named Jaime “Hummer” Gonzalez Duran, who ran all aspects of the drug trafficking business — including enforcement to collect debts or eliminate rivals — between Reynosa, Mexico and McAllen, Texas, according to interviews with FBI and Drug Enforcement Administration officials.

For at least two years before the weapons seizure, the DEA's Houston office had been after Gonzalez and his cell, in an investigation known as “Dos Equis.”

Gonzalez and 11 associates were indicted on federal trafficking charges in September, but remained free as fugitives in Mexico. About the same time, the FBI issued an alarming intelligence alert to all law enforcement personnel in the McAllen area that Hummer was stockpiling weapons with which to attack American law enforcement officers.

“Each cell leader has been personally instructed by Hummer to engage law enforcement with a full tactical response should law enforcement attempt to intervene in their operations,” the memo said.

But those plans were thwarted.

Shortly after the FBI sent out its alert, DEA agents told their Mexican counterparts where they believed Gonzalez was hiding out, said Houston-based DEA Special Agent Violet Szeleczky. Within the same 24-hour period, from November 6 to 7, Mexican authorities captured Gonzalez and took over his massive weapons cache.

No violence against American law enforcement in Texas has been reported.

For more GlobalPost Dispatches on the border troubles:

Analysis: Mexico a failing state?

The cross-border bullet trade

To live or die in Mexico

Related Stories