Evidence is collected at the scene where eight people were found dead in a tractor-trailer loaded with multiple others, outside a Walmart store in stifling summer heat in what police are calling a horrific human trafficking case, Sunday, July 23, 2017, in San Antonio. (AP Photo/Eric Gay)

Evidence is collected at the scene where eight people were found dead in a tractor-trailer loaded with multiple others, outside a Walmart store in stifling summer heat in what police are calling a horrific human trafficking case, Sunday, July 23, 2017, in San Antonio. (AP Photo/Eric Gay)

CHICAGO (AP) — It could take months for investigators to determine what preceded the deaths of at least nine people found with dozens of ailing individuals in a tractor-trailer discovered outside a Walmart in San Antonio, Texas, in what authorities are calling an immigrant-smuggling attempt gone wrong.

But previous cases of smugglers using similar trucks to move human cargo shed light on the dangerous method of human trafficking — and how it can quickly turn fatal.

Here’s a look at how smugglers deploy and use large trucks to move people:

HOW COMMON IS HUMAN TRAFFICKING BY TRUCK?

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Border officials have reported an uptick in the number of people-smuggling incidents using tractor-trailers. That included one on July 7, when Border Patrol agents in Laredo, Texas, found 72 people from Mexico, Ecuador, Guatemala and El Salvador locked inside a trailer. Weeks before, they’d rescued 44 people from Mexico and Guatemala discovered after police stopped an 18-wheeler near one of the city’s international bridges.

Whether this recent increase is a trend and what might explain it is hard to know. A recent report from European-based global -risk group Verisk Maplecroft suggests that a harder line on border security by the Trump administration might be leading migrants to accept the risks of more dangerous smuggling methods.

By far the most notorious and best documented case occurred in 2003, when 19 of about 100 people being smuggled in a truck trailer in south Texas died of heat-related injuries; that included a 7-year-old boy. More than a dozen smugglers were convicted in that case, including the American commercial driver at the wheel, Tyrone Mapletoft Williams, and the purported head of the smuggling ring, Karla Patricia Chavez-Joya, a Honduran national.

WHERE ARE THE IMMIGRANTS FROM?

Transportation by truck is often one of the final steps in a process that can begin months before somewhere in Mexico or more than a thousand miles from the U.S.-Mexican border in Honduras or Guatemala. A review of court documents in other cases indicates the tractor-trailers are often brought in only after Mexicans and Central Americans arrive by train, bus or car to the Mexican-U.S. border region — and then slip into the U.S. by foot or by raft across the Rio Grande.

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In the 2003 case, the pickup site for the immigrants was near Harlingen, Texas, about 20 miles (32.19 kilometers) from the U.S.-Mexican border. The plan was to drive the tractor-trailer through an immigration checkpoint 50 miles (80.46 kilometers) away on Highway 77 near Sarita, Texas; once through the crossing, the immigrants were to be transferred to separate vans bound for Houston.

WHY TRUCKS?

The objective of immigrants who make it undetected across the border typically isn’t to remain in that border area. Most hope to make it to large U.S. cities, like Chicago or New York, where they may have jobs or family waiting for them. That’s where the trucks come in. Smugglers know there are hundreds and thousands of immigrants desperate to get away from the border as fast as possible. And they see the money-making opportunity. The more people they can move at one time, the more the profit.

In the 2003 case, the smugglers actively sought non-Hispanic, American drivers who they believed would be less likely to raise suspicions and more likely to make it through the Sarita checkpoint. Tyrone Williams, a licensed truck driver from New York, fit that description. Just before picking up his human cargo, Williams had hauled milk products from New York in his refrigerated truck.

HOW DO THEY DIE?

Dehydration, hyperthermia, suffocation, and mechanical asphyxia have been among the causes of death in truck cases. In the 2003 case, Williams’ trailer was equipped with a refrigeration unit that enabled him to haul milk earlier at 35 degrees Fahrenheit (1.67 Celsius) from New York to Texas. But when it came to his human cargo, he didn’t turn it on. The immigrants remained silent as Williams was waived through the checkpoint, but they soon after began banging on the sides of the trailer as it became increasingly hot and increasingly hard to breathe. Finally, fearing detection, Williams unhooked the trailer at a truck stop in Victoria, Texas, and drove off. An appellate court later described the scene inside: “Bodies, both dead and living, were stacked in a pile in the trailer. Some of the aliens were standing behind the pile. The aliens were stripped down to their underwear and were sweating.”

CHARGES

Because the crime involves the crossing of international and of state borders, it’s often federal authorities who prosecute human traffickers. The available charges range from conspiracy to aiding and abetting the transporting of unlawful aliens resulting death. Maximum sentences can range from a few years behind bars and to the death penalty. Prosecutors did initially indicate they would seek the death penalty for Tyrone Williams. But in 2012, a federal judge sentenced him to more than 30 years in prison without the possibility of patrol.