Far more people are smuggled in cars. In the current fiscal year, which began Oct. 1, nearly 2,000 migrants have been caught in cars, compared with about 225 in commercial trucks, according to the Border Patrol. (In the previous fiscal year, those numbers were 3,400 and 369, respectively.)

But trucks provide several advantages over cars for smugglers and migrants.

One is bulk. One eighteen-wheeler trip is often the work not of a single smuggler but of several working together, who load four, five or six groups of 20 or so migrants into a trailer. In the San Antonio case, one immigrant believed that up to 200 people had been inside at one point. They had been handed tape with different colors so their handlers could keep track of which groups went with which smugglers at the drop-off point.

“Usually if you’re in those big vehicles, it’s trying to coordinate large groups and move people around,” said Jeremy Slack, a migration expert and professor at the University of Texas at El Paso.

Another benefit is evasion. So many trucks in the Southwest are moving goods to and from Mexico that the Border Patrol cannot possibly check all of them. To minimize the risk of inspection, smugglers will try tactics like the rotting watermelons, which did not work. If a truck is refrigerated, the driver will often turn the cooling system off before reaching the checkpoint so that inspectors will not get suspicious when the driver claims the truck is empty.

Once past the checkpoints, the trucks are bound for major cities, such as Houston and San Antonio, that have become hubs for human and drug smuggling. At the drop-off points, the migrants are put into smaller vehicles for the next leg of their journey.

But the ad hoc smuggling system is fraught with delays and faulty equipment, as well as misjudgments about how long people can survive packed into often-unrefrigerated metal boxes in the Texas heat.