Reporter: Craig McMurtrie
Mexican drug cartels are expanding operations in the USA, where authorities are struggling to respond.
FBI called in: A soldier takes photos at the scene where the American couple were killed last year.
ALI MOORE, PRESENTER: The shocking violence of the Mexican drug cartel shows no sign of easing, with more deaths over the weekend.
Last year alone, 15,000 Mexicans were slaughtered in the cartel battles, waged against each other and the Mexican Government. And what isn't so widely reported is the worrying increase in cartel activity on the US side of the border.
The Obama administration has threatened to respond with overwhelming force if the bloodshed spreads into America.
But as Craig McMurtrie discovered in this Lateline investigation, despite throwing more resources at the problem, US authorities are struggling to choke off the cartel's expanding drug and gun-running networks.
CRAIG MCMURTRIE, REPORTER: La Joya, southern Texas.
12-year police veteran lieutenant Julian Gutierrez answers a call for backup. A routine stop for a traffic violation has taken a suspicious turn. The three occupants, all men, are separated and questioned.
In a poor community, they're carrying fat rolls of cash.
The police suspect it could be cartel money for cars. The cash is confiscated. Two men are taken for interrogation; a third has an active arrest warrant.
JULIAN GUTIERREZ, LA JOYA POLICE: I know the driver's going to go in for - he's wanted for possession of marijuana at this time.
CRAIG MCMURTRIE: Drug-running is a daily occurrence in La Joya. Mostly it's marijuana and the quantities are massive. Last month there was an hour-long car chase. A suburban four-wheel drive had run a stop sign. The driver sped off toward the Rio Grande River, the border between the US and Mexico.
Local police chief Jose Del Angel says three patrol cars were damaged. And by the time the first patrolman pulled up, hundreds of kilograms of marijuana were already being unloaded by more than a dozen men.
The suburban is dumped in the river. A boat carrying the contraband speeds away, only to turn back when confronted by Mexican soldiers on the other side.
JOSE DEL ANGEL, LA JOYA POLICE CHIEF: And they said the aluminium boat, (inaudible) and they flee again, they escape capture again.
CRAIG MCMURTRIE: Do you have the resources that you need?
JOSE DEL ANGEL: No, we don't have the resources to be fighting these kind of people. As you see, there was about 15 to 20 guys. How can you be fighting a war with those type of individuals. They're not afraid of us, they're not.
CRAIG MCMURTRIE: In the police vault next to the cells, blocks of marijuana are piled up from this month's successful seizures. It's estimated 60 per cent of the cartel's income comes from marijuana.
JULIAN GUTIERREZ: So we're looking at over 1,000 pounds here that we have confiscated, you know, and that's not generally - we've burned already about 3,000 pounds last month.
CRAIG MCMURTRIE: It's brazen. Sometimes the drug runners don't even bother to hide their load, and it isn't always drugs. Here a people smuggler jumps from his moving car. The sedan rolls on and catches fire. In the back seat and boot police find half a dozen Mexican nationals who'd paid to be smuggled into America.
They paid across the river and are then often held until their US connections pay a ransom.
JULIAN GUTIERREZ: They bring the boats all the way up to here and they load it. Matter of fact, look at that. Lo and behold, look at that.
CRAIG MCMURTRIE: There's a boat there.
JULIAN GUTIERREZ: See, there you go.
CRAIG MCMURTRIE: So they would use boats like this.
JULIAN GUTIERREZ: Yeah, they use boats like this and carry people over, carry dope over. Usually this is a good boat right here for dope.
CRAIG MCMURTRIE: For obvious reasons, most movement across the river happens at night. Sometimes the locals hear gunfire from the Mexican side.
So just to get our bearings straight, we're in the United States here, this is the Rio Grande River and that is Mexico?
JULIAN GUTIERREZ: Yes.
CRAIG MCMURTRIE: It really is that close.
Washington has committed over a billion dollars to try and secure the border. Security here at the Hidalgo Bridge crossing's never been tighter. There have never been so many border patrols. But the flow of people smuggling and drug running seems inexhaustible.
The cartel's influence now reaches well beyond the border here in the Rio Grande Valley. It's estimated their drug distribution networks now extend to 270 US cities and towns.
And they're not just smuggling marijuana into the US; they're growing it in the US on public lands, in massive quantities. From California to Virginia, the cartels are turning some of America's most pristine wilderness and national park areas into giant-sized pot farms.
In the mountains north of Los Angeles, members of the sheriff's department narcotics bureau have found evidence of a hastily-abandoned camp in a national forest. Beyond, there's a hidden marijuana garden.
This is a relatively small find, only 1,000 plants. Armed guards and booby traps are not unusual. Several law enforcement officers have been shot on such missions.
SYLVIA LONGMIRE, DRUG CARTELS ANALYST: What the Mexican cartels do is they bring illegals in from Mexico to work these grows, and they give them guns, they give them the equipment and they say, "This is what we want you to do." And it works very much like a terrorist cell structure where the people working and this grow and that grow, they don't know who they are. Most of the time they don't even know who they're working for.
CRAIG MCMURTRIE: To protect their mountain-top gardens, their US stash houses and their turf in the deadly drug war in Mexico, the cartels need weapons. Lots of them. And here, once again, America supplies.
Last November, the front window of this Houston gun store was smashed. Closed circuit cameras recorded the daring raid. A red suburban ignored the empty patrol car parked outside. It backed up to the front window, two men got out. One used a rock to smash the glass. When they raced off moments later, they had a dozen Glock handguns and half a dozen assault rifles.
JIM PRUETT, GUN STORE OWNER: So we believe it was an order by the cartel to buy - to secure certain weapons by gangs who are the cartel henchmen, if you would, and they pulled it off pretty successful. We haven't caught 'em to date.
CRAIG MCMURTRIE: For store owner Jim Pruett it was a first, but there are hundreds of licensed gun dealers in Houston and over 100,000 across America. According to US Government figures, 80 per cent of the thousands and thousands of firearms seized in Mexico are from the United States. Nearly half of those, from Texas.
Not all are stolen. The cartels also recruit US citizens with clean records to go store to store buying guns for them.
CHARLIE HOUSER, ATF NATIONAL GUN TRACING CENTRE: It's not only right along the border in a few little spots. It extends all the way through the United States wherever the cartels may be trafficking their drugs.
CRAIG MCMURTRIE: In the West Virginia countryside, the US federal agency responsible for regulating firearms has a unique library. Instead of books, the ATF has shelves full of thousands of seized or modified guns.
AT OFFICER: Well, this is a military-style rifle. The ammo is, you know, readily-available.
CRAIG MCMURTRIE: The bureau also has piles and piles of paperwork, thousands of trace requests for firearms, millions of records from gun dealers going out of business and everything is processed by hand, because under the critical eye of America's gun lobby, the ATF is barred by Congress from keeping a computer database or registry of firearms.
The Bureau has no idea of the volume of gun sales in the United States and no-one truly knows how many are crossing the border.
WILLIAM MCMAHON, ATF DEPUTY ASSISTANT DIRECTOR: It is harder for the cartels to get some firearms. You know that certain cartels are having a harder time supplying their soldiers, for lack of a better word. But you're absolutely right: we don't know the totality of it, both in Mexico and in the US.
CRAIG MCMURTRIE: So far in La Joya, while they're armed, the human smugglers and drug runners haven't fired on police. On American soil, the cartels prefer to run rather than fight. The hard choice ahead is that if Washington does decide to up the ante, they could stop running and life for La Joya's finest will get a lot more dangerous.