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Walls, fences, boots on the ground and camera-equipped blimps — all have been pitched as ways to stop the illegal flow of people and drugs over the southern border into Texas and beyond.

But overlooked in the rush to secure the border, score political points and stymie the threat of spillover violence is the river of handguns, rifles, assault weapons and bullets of all shapes and sizes that continues to flow southward from the United States to Mexico, arming some of the very people officials say present the greatest threat to Americans.

From 2009 to 2014, more than 73,600 guns seized in Mexico were from the United States, according to a 2016 report from the Government Accountability Office, the congressional watchdog over the federal government. More than 13,600 were confirmed by the Bureau of Alcohol, Tobacco, Firearms and Explosives to have originated in Texas. But that figure could be higher — the report also states that because of factors like altered serial numbers on weapons and incomplete information on records, the states of origin could only be traced for about 45 percent of the U.S. total.

That continues a years-long trend of Texas as a major supplier of weapons used in crimes south of the Rio Grande, according to the ATF.

Some of the weapons are stolen and smuggled south, but most are bought at retail gun stores, pawn shops or gun shows that — at least in Texas — occur weekly in major cities like Houston, Dallas and San Antonio.

The weapons range from hand cannons to rifles to military-grade weapons, but the preferred guns are long rifles including the semi-automatic varieties such as the AR-15 and AK-47, according to a 2016 GAO report. They are easily converted to fully automatic machine guns, the report notes.

Mexico's gun laws partly fuel the demand. Although private gun ownership is allowed, buying a weapon legally is difficult. Mexicans must get a permit from the Mexican army and can buy guns only in the country’s lone gun store in Mexico City. And that seller, the Directorate of Arms and Munitions Sales, can’t advertise its goods, according to an August CBS report.

Shipping guns and ammunition in from the United States is easier, and there are few lines of defense. On the U.S. side, the ATF is the lead agency assigned to control the sale of guns and their illicit transfer to Mexico and Central America.

“ It’s a vicious cycle. We send our guns down south, and they send their drugs up north. ” — State Rep. Rafael Anchia, D-Dallas

But in the country's southern states, the agency's staff is less than 20 percent of that of the U.S. Border Patrol, whose ranks in Texas, Arizona, California and New Mexico exceed 17,500.

“ATF's number of special agents generally increased from fiscal years 2004 through 2010, but decreased by about 6 percent (from 2,562 to 2,399) through fiscal year 2013, which represents the lowest number of special agents in 8 years,” the GAO reported in 2014. “According to ATF management officials, ATF was unable to hire agents because its funding did not keep pace with the cost of employee salaries and benefits.”

Turf wars

It's not just gunrunners making ATF's job difficult. The 2016 GAO report stated that — although they share a mission — agents from ATF and Immigration and Customs Enforcement don’t see eye to eye on how best to detect and deter weapons moving south.

“ATF and ICE disagree on the extent to which trace data on firearms seized in Mexico collected through eTrace should be shared to support ICE firearms trafficking investigations,” the report states. “According to an ICE assistant deputy director, these firearms trace data from Mexico are currently only shared on a limited basis with ICE. Several ICE officials expressed an interest in obtaining access to these data and indicated that this access would enhance their ability to identify methodologies used by firearms traffickers and trends in criminal activity along the Southwest border.”

Asked about the report in March, ICE spokeswoman Sarah Rodriguez said it also noted improvements ATF and ICE have made in their coordinated efforts and that the agencies would continue to work to streamline investigations.

“The investigative arm of the agency, Homeland Security Investigations, has already engaged with ATF to review potential solutions to further enhance coordination between the two agencies in support of this effort," Rodriguez, who is based in Washington, D.C., said in an email. “In Fiscal Year 2015, HSI initiated nearly 3,000 investigations targeting the trafficking or illegal export of materials used to manufacture weapons of mass destruction, chemical, biological, radiological, and nuclear materials, military equipment and technology, controlled dual-use commodities and technology, firearms, and ammunition.”

A later follow up with ICE in El Paso asking for details about how the miscommunication has affected specific cases in Texas didn’t yield a response. The agency initially agreed to an interview with The Texas Tribune about the issue. But after the GAO report was mentioned, a spokesperson for ICE in El Paso instead re-sent the prepared statement emailed in March.

So close to Texas

Illicit arms trafficking from Texas has for years contributed to the carnage in Mexico, where tens of thousands have died since a full-scale war between law enforcement and rival cartels began in 2005.

But convincing lawmakers to scale back gun rights in Texas because of the connection to violence in Mexico has been, so far, a non-starter in a state that has adopted “Come and Take It” as an unofficial motto.

“ What you’ll see is that they’ll go to a gun store and ask for five of one rifle and a bunch of magazines. ” — Justin Williamson, Texas State Rifle Association

If anything, state legislators are headed in the other direction. In 2015, they expanded gun rights by passing laws allowing 21-year-old college students with licenses to carry concealed weapons on some college campuses and licensed carriers to tote their guns in plain sight, a practice commonly referred to as open carry.

Proponents of the Second Amendment have fought back against claims that gun shows, which are held in major Texas cities at least once a month and advertised on billboards along every major highway, are the major source of guns that end up used in crimes south of the Rio Grande.

The GAO's 2016 report states that most of the guns headed south were purchased legally at gun shows and subsequently smuggled into Mexico.

By law, firearms dealers must be registered and licensed by the federal government. As of 2015, Texas had more of those sellers, 10,856, than any other state. (There were an additional 131 applications pending — another nationwide record – as of the same date.) But some decry what gun control supporters call the “gun show loophole," which they argue allows dealers to sell without a license or without conducting background checks on the buyer.

The Texas State Rifle Association, the state affiliate of the National Rifle Association, said the so-called loophole is a myth.

“The people who buy tables at gun shows, ATF requirements requires that if [they are] going to purchase firearms for the use of selling or for making a profit or for running a business, you have to register for a [federal firearms license], which requires that you run background checks,” said Texas State Rifle Association spokesman Justin Williamson.

The problem, according to the GAO report authors, is how easy it is for unlicensed sellers to obtain and resell weapons.

“Firearm trafficking organizations also frequently obtain firearms from unlicensed private sellers in secondary markets, particularly at gun shows and flea markets or through classified ads or private-party internet postings, according to ATF officials,” the report states.

State Rep. Rafael Anchia, D-Dallas, said expanding background checks isn’t as unpopular as some gun rights proponents assume. He cited an October 2015 article by fact-checking site Politifact finding that about 90 percent of Americans, including about 75 percent of NRA members, supported background checks for all purchases.

“Polls continue to show a strong tilt in favor of background checks prior to all gun purchases,” the authors wrote. “Then again, the popularity of this idea may bear watching. Other 2015 polls found as few as 84 percent of respondents backing the universal checks.”

Anchia pre-filed legislation in November that would expand background checks in Texas and include those private purchases he said can be done anywhere, including the parking lot of a store. His House bill 259 would expand the definition of a “gun show” to include a place other than a permanent retail store, including a "driveway, sidewalk or walkway, parking lot, or other parking area.”

The bill would make it a class A misdemeanor if a seller didn’t run a background check or maintain a record of the transaction.

“It’s a vicious cycle. We send our guns down south, and they send their drugs up north,” he said. “You can buy a weapon that can stop a heavy, armored vehicle without a background check in Texas. That should be troublesome for everybody.”

But Anchia is in the minority party, and Republicans are returning to Austin next year looking to pick up where they left off in 2015, when they passed open and campus carry bills. State Rep. Jonathan Stickland, R-Bedford, pre-filed legislation that would allow non-licensed gun holders to carry their firearms in public, a measure commonly referred to as “constitutional carry.”

Anchia said he remains determined to at least have a discussion on the issue.

“You’re never going to get a hit unless you step up to bat,” he said.

Williamson accepted that a problem exists with straw purchasers — a buyer that purchases weapons under the guise of personal use but instead sells them to others who are prohibited from buying themselves. Felons, people under indictment, undocumented immigrants, some legal immigrants and people subject to a restraining order aren't legally able to buy a gun.

In one high-profile Texas case, a Houston man was stopped in South Texas by police officers in 2013 and found with 35 military-style weapons bound for Mexico. The driver, Angel Aquino Pineda, was just one of several members of a gunrunning ring that authorities convicted after making hundreds of trips during their years-long operation, the Houston Chronicle reported.

Williamson said that most licensed dealers do their due diligence and report people they think might be straw buyers.

“What you’ll see is that they’ll go to a gun store and ask for five of one rifle and a bunch of magazines,” he said. But if it’s not a major bust for law enforcement, he said, prosecutors won’t spend the resources building a case if it only yields a minor punishment or doesn't lead to a larger cache of weapons.

As proof, Williamson cited the most famous example, the botched Fast and Furious campaign, where agents let small-time gun smugglers into Mexico in the hopes of nabbing larger operatives.

This story is part of Tribune's yearlong Bordering on Insecurity project.