They've brought logistical know-how, using tractor-trailer trucks to carry ever larger loads of people and charging higher prices, as much as $30,000 per head for migrants from Asia and Africa who seek to get to the United States.

They've also brought an unprecedented level of intimidation and violence to the trade. Los Zetas or their allies often kidnap and hold for ransom poor migrants who try to operate outside the system. If relatives don't wire payment, the migrants sometimes are executed and dumped in mass graves or press-ganged into jobs with the criminal group.

Nearly a year ago, Zetas gunmen were implicated in the slaughter of 72 migrants at a ranch near San Fernando in Tamaulipas state, barely an hour and a half drive from Brownsville, Texas.

Other mass graves discovered in northern Mexico also may be the work of Los Zetas pushing to control smuggling to the United States.

Alejandro Solalinde, an activist Roman Catholic priest who runs a migrant shelter in the town of Ixtepec, in Mexico's Oaxaca state, said Los Zetas had been merciless with migrants.

"Los Zetas control the trafficking of persons," he said. "They are crueler and kill more easily. ... They are voracious. They ask for more and more and more money."

Even with an apparent drop in the numbers of migrants moving through Mexico, people smuggling is a huge business.

A U.N. report last year titled "The Globalization of Crime" estimated that Mexican smugglers rake in $6.6 billion annually from the 3 million Latin Americans who are taken across the southern U.S. border each year. Two weeks ago, Mexico's National Institute of Migration said that 6 out of 10 migrants paid traffickers to help them cross the U.S. border, while 43 percent used them to traverse Mexico as well.

While such estimates routinely are disputed, officials acknowledge that drug gangs have found a new revenue source in human trafficking. Mercedes Gomez Mont, the top immigration official in Chiapas state, which borders Guatemala, cited criminal investigations by the Mexican Attorney General's Office for her declaration that while "organized crime derives its greatest income from drug trafficking ... in second place is human trafficking."

The ascendancy of Los Zetas in migrant smuggling, formerly the preserve of relatively small independent operators known as "coyotes" who smuggled groups of 20 or fewer migrants north, has transformed the business.

Mexican officials report regularly finding tractor-trailer trucks loaded with as many as 250 migrants in their holds. The heavily armed drivers, who travel with escort vehicles, make payoffs at police and immigration checkpoints.

Two such tractor-trailers were detained at a checkpoint May 17 near the Chiapas state capital of Tuxtla Gutierrez. X-ray equipment revealed the ghostly outlines of human cargo, and when officials opened the holds they found 513 people from El Salvador, Ecuador, China, Japan, Guatemala, India, Nepal, Honduras and the Dominican Republic - a United Nations gathering of migrants.

Each migrant had paid at least $7,000 to travel through Mexico to the United States, Mexico's attorney general said, making the cargo worth more than $3 million.

It was the largest such discovery in Mexico's history, and it underscored the complicity of authorities in the trade. The two tractor-trailers had passed at least two previous checkpoints in Chiapas before the discovery.

"You pay as you go," said a U.S. law enforcement official based in Mexico City, who spoke on the condition of anonymity. "That money is being dropped off all along the route."

Solalinde, the priest-advocate, said he'd been shown the tightly rolled wads of cash the trafficking convoys passed out.

"It is incredible how much money can be in such a small roll," he said.

Trucks carrying large numbers of migrants have been found repeatedly this year. On Jan. 26, authorities found 219 migrants in the back of a tractor-trailer near San Cristobal de las Casas in Chiapas. On June 12, another 202 migrants were discovered in Veracruz state, and 117 more were found June 23 in Oaxaca state.

Los Zetas now have intermediaries in Guatemala, Honduras and El Salvador who recruit migrants and send them along established routes northward.

"If you want to arrive safely at your house in the United States, you have to pay these coyotes between $7,000 and $10,000," said Patricia Villamil Perdomo, who was the Honduran consul in Tapachula, a Mexican city near the Guatemalan border, until mid-June, when she quit after receiving written threats signed by "Z," or Los Zetas.

Villamil said coyotes told her that they now must pay a "fee" to the Zetas if they worked independently and passed migrants through turf controlled by the gang.

"Everything is passing through their hands," she said of Los Zetas.

Zetas operatives are known to be strong in the southern border state of Tabasco, in Veracruz along the Gulf Coast and in Tamaulipas, which abuts Texas. But they're also in Oaxaca and Chiapas.

In Arriaga, the southernmost rail yard in Mexico, residents said they often saw pickups with tinted windows and license plates from Tamaulipas, the home state of Los Zetas.

As organized crime takes over migrant trafficking, its power to corrupt has become evident. In May, President Felipe Calderon fired seven top officials of the national immigration agency, which had been wracked by allegations that officers in northern Mexico had turned over Central American migrants to mobsters.

Los Zetas profit not only from moving better-off migrants but also by preying on poorer Central Americans who try to pass through Mexico on their own. Masked gunmen in cahoots with train conductors and brakemen halt trains in remote spots and take hostage the migrants who ride atop the boxcars and tanker wagons.

The hostages, often dozens, or scores, are taken to ranches, where they're held while gangsters call relatives in the United States and demand ransom. The longer it takes for payment to arrive, the more the kidnappers charge.

"We call it migrant extortion kidnapping," said the U.S. law enforcement official, who added that hostages are given two choices: Go to work for Los Zetas or find a way to pay thousands of dollars in ransom.

Sometimes the hostage-takers are members of allied criminal networks.

"The Zetas, unlike other cartels that have centralized commands, operate with franchises. They have arrangements with local and regional networks," said Rodolfo Casillas, an immigration expert at the Latin American Faculty of Social Sciences, a hemispheric research and teaching academy with a branch in Mexico City.

The U.S. official concurred: "The upper echelon of the Zetas tell the middle echelon: 'These are the crimes that you can do with our authorization to generate revenue for the organization.' " Midlevel crime bosses then coordinate criminal activity in their assigned regions.

Migrants who survive abduction say the gunmen operate with a swagger.

"The kidnappers say they are with Los Zetas, they are really well-armed and that the authorities are in cahoots with them," said the Rev. Heyman Vazquez Medina, a priest who runs a local shelter and has debriefed hundreds of migrants.