SAN DIEGO >> When Thomas Homan, the acting director of U.S. Immigration and Customs Enforcement, was awakened Sunday morning with news that migrants were found dead inside a sweltering tractor-trailer outside a San Antonio Walmart, his mind flashed back to 2003, when he stood at the back of a truck about 120 miles (200 kilometers) southeast of San Antonio that carried 19 dead migrants.

“It is sad that 14 years later people are still being smuggled in tractor-trailers,” he said. “There still isn’t water, there still isn’t ventilation. These criminal organizations, they’re all about making money.”

The striking similarities of the Texas tragedies demonstrate how smugglers have found a durable business model carrying large groups — often in big rigs — through an elaborate network of foot guides, safe house operators and drivers. A criminal complaint about Sunday’s discovery that 10 were dead and dozens injured in the truck opens a window on their degree of sophistication and organizational muscle: passengers had color-coded tape to split into smaller groups; and six black SUVs awaited them at one transit point to bring them to their destinations.

Big rigs emerged as a popular smuggling method in the early 1990s amid a surge in U.S. border enforcement in San Diego and El Paso, Texas, which were then the busiest corridors for illegal crossings. Before that, people paid small fees to mom-and-pop operators to get them across a largely unguarded border. As crossing became exponentially more difficult after the 2001 terror strikes in the U.S., migrants were led through more dangerous terrain and paid thousands of dollars more.

Guadalupe Correa-Cabrera, a political scientist who teaches at University of Texas, Rio Grande Valley, said migrants she interviewed last year in South Texas paid $2,000 to $3,000 more to ride in the crammed tractor-trailers, considering them more effective, faster and safer than walking through the desert to a pickup point far from the border. Hundreds of border crossers perish each year in the desert, getting lost and dehydrated in extreme heat.

The growing use of trucks coincided with increased trade with Mexico under the North American Free Trade Agreement, allowing smugglers to more easily blend in with cargo, particularly on Interstate 35 from Laredo, Texas, to San Antonio, Correa-Cabrera said. Walking in the open desert more easily exposes them to U.S. Border Patrol agents.

Women, some carrying children, think they are less likely to be raped on a truck than in the open desert because there are more witnesses, Correia-Cabrera said. Riding in a big rig, she said, is “the VIP treatment.”

For smugglers, the advantage of tractor-trailers boils down to scale.

“It’s like any other business: the more they move, the more profit they make,” Homan said. “Rather than taking four in a car, the profit margin on tractor-trailers is a lot more.”

Truck drivers are low-level cogs in a big machine, recruited in the U.S. at casinos and other places where smuggling organizations look for people who are down on their luck, desperate for quick cash and disinclined to ask questions.

James Matthew Bradley Jr., who made an initial court appearance today in San Antonio on smuggling charges, told authorities he was delivering what he thought was a sold vehicle from Schiller, Iowa, to Brownsville, Texas, and that he didn’t know what was inside, according to the complaint. He said he was given no deadline or address to deliver the truck.

Other guides take migrants across Mexico by bus. Others join them on a raft across the Rio Grande or through the desert to a hideout or to a nearby house where they may wait days or weeks. Eventually smuggling organizations get them to major cities like Phoenix, Houston or San Antonio.

“I have to imagine that their winning percentage is really, really high,” said Adam Isacson of the Washington Office on Latin America, a human rights advocacy group. “Whatever reputation they lose from episodes like this, their profit margins are still high enough to make it work. Otherwise people wouldn’t pay.”