ARRIAGA, Mexico — One of Mexico's most powerful criminal gangs has muscled into the migrant-smuggling racket, changing what had been a relatively benign if risky industry of independent operators into a centralized business that often has deadly consequences for those who try to operate outside it.

Los Zetas, who earned a reputation for brutality by gunning down thousands of Mexicans in the ongoing battle for drug-smuggling routes to the United States, now control much of the illicit trade of moving migrant workers toward the U.S. border, experts in the trade say.

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.