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.