A revealing X-ray image showed hundreds of migrants crammed into two trailer trucks. Authorities said there were a total of 531 people from all over the world.
Officials said they paid as much as $7,000 for the trip into the U.S. The trucks were stopped in the Mexican state of Chiapas, and the drivers were detained.
Most of the migrants came from Mexico, South and Central America, but there were some from as far away as Nepal, India, China and Japan.
Mexican authorities returned about 400 migrants to their native Guatemala, while 113 migrants are still being held because they are from nations that don't share borders with Mexico or are Guatemalan minors or women who require special treatment.
The remaining migrants include 47 from El Salvador, 32 from Ecuador, 12 from India, six from Nepal, three from China and one each from the Dominican Republic and Honduras, as well as Guatemalan minors and women.
The Associated Press contributed to this story.