SASABE, Mexico – Weighed down by 50-pound sacks of marijuana, they hike through the desert for days to reach remote drop-off points in the United States, then sneak back across the Mexican border.
They are seldom illegal immigrants, but drug runners, physically fit and able to carry heavy loads for long distances. In some cases, it's been the family business for generations.
Lately, they're getting caught running more drugs than ever — often over the same desert routes used by undocumented migrants — with the U.S. Border Patrol reporting a sharp increase in the amount of marijuana seized.
The Border Patrol's Tucson, Ariz., sector confiscated almost as much marijuana in the first 7 1/2 months of this fiscal year as they did all of last year, and seizures are up all along the border.
"These are often second- and third-generation smugglers. This is what they do for a living, and once they deliver a load, they simply go back" into Mexico, said Gustavo Soto, a spokesman for the Tucson sector of the Border Patrol.
In Soto's sector, agents seized 441,482 pounds of marijuana between Oct. 1 and May 17, compared to about 489,000 pounds for all the preceding fiscal year ending in September. Across the entire border, the Border Patrol has seized 957,638 pounds of marijuana this year, up from 755,914 pounds in the same period last year.
That figures include drugs smuggled into the United States in vehicles driven into the desert, but increasing border security measures like fences and vehicles barriers appear to be making that more difficult.
Soto said the backpackers — known as narcotics "mules" — "wander in the desert for anywhere from two to possibly even four days until they get to their destination."
The drug runs are so difficult that Steve Robertson, a U.S. Drug Enforcement Agency special agent who has worked along the Texas border, says the average migrant couldn't make them.
Soto attributes the rise in seizures to increased border security, including sensors, cameras and a variety of desert vehicles.
But Mexico's Attorney General, Daniel Cabeza de Vaca, said cartels are also becoming more reliant on income from marijuana. "We know that marijuana is becoming more and more important (for the cartels) and that cocaine is on it's way down," he said.
Changing U.S. drug habits — like the increasing popularity of synthetic drugs, such as methamphetamines — also have cut into cocaine's profitability, experts say.
On Monday, Mexican federal agents announced one of the biggest marijuana seizures in recent memory: 8.4 tons of pot, packed among fertilizer inside a truck bound for the U.S. border.
Border Patrol cocaine seizures are down to 5,260 pounds so far this fiscal year, from the 6,406 pounds seized in the same period last year.
The figures do not include drugs seized at formal border crossing points, which are the responsibility of U.S. Customs.
Customs and Border Protection spokesman Todd Fraser said the surge in marijuana seizures may be short-lived, and may force traffickers to use other tactics.
"In the short term, you may see an increase," he said, "but I think over the long term there may be a decline, as these drug trafficking organizations say 'Hey, they're intercepting our loads. It's not worth it smuggling over these traditional routes.'"
The recent discovery of smuggling tunnels under the border may be a sign of the traffickers' desperation, he said.
"We look at this as a success," Fraser said. "We're literally forcing these people underground."
The two phenomena — illegal immigration and marijuana smuggling — may overlap only in geography and tactics, officials say.
"We have no evidence that (drug traffickers) are getting involved in migrant smuggling," Cabeza de Vaca said. Soto noted that only on "rare occasions" are immigrants found carrying drugs, in some cases as a way to pay the fees smugglers charge for guiding them through the desert.
But drug traffickers and immigrant smugglers do have one thing in common: They are increasingly likely to attack U.S. personnel who try to detain them, U.S. officials say.
Fred Patton, chief ranger of Organ Pipe Cactus National Monument — a U.S. park on the Arizona border — says the threat of violence is so great that at least 20 percent of the park is closed to public, and there are some areas where even rangers cannot go.
"The biggest threat to our people is illegal activity," Patton says.
Soto said traffickers and smugglers have used every type of weapon to hit back at agents.
"The frustration level is starting to increase because of our effectiveness," he said, citing attacks that include rock throwing, shootings, and even cases where smugglers have tried to run over agents with their vehicles.