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 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.