SAN ANTONIO, Tex. — The San Antonio authorities discovered at least eight bodies in a tractor-trailer in a Walmart parking lot on Sunday morning in what the police chief called a “horrific” human trafficking crime.
The chief, William McManus, said at a predawn news conference that a store employee making the rounds late Saturday night was approached by someone from the truck “asking for water.” The employee returned with the water and called the police, who found the bodies at the back of the truck.
An expert on border enforcement and migrant deaths called the trucks “mobile ovens.”
“Those things are made out of steel and metal,” the expert, Néstor P. Rodríguez, a sociology professor at the University of Texas at Austin, said on Sunday. “Yesterday in Austin, it was like 96 degrees at 9:30 in the evening. Even if the cooling system is on in the tractor-trailer, it’s just too hot.”
The eight fatalities were believed to have been caused by heat exposure and asphyxiation, a spokesman for the San Antonio Police Department said by email. The bodies were taken to the Bexar County Medical Examiner’s Office to determine the cause of death. Details about the victims were unavailable.
At least 38 people were in the trailer, the fire chief, Charles Hood, said at the news conference. About 20 were taken to hospitals in “extremely severe” or critical condition, he said. Eight others had injuries that were not life-threatening.
Two of those found were “school-age children” and the others were in their 20s and 30s, the chief said. The two youngest of those injured were 15, the spokesman said in the email.
The driver, who was not immediately identified, was in custody and will be charged, the top federal prosecutor in the San Antonio area said in a statement on Sunday.
“These people were helpless in the hands of their transporters,” said the prosecutor, Richard L. Durbin Jr., the United States attorney for the Western District of Texas. “Imagine their suffering, trapped in a stifling trailer in 100-plus-degree heat.”
Chief McManus said that “we’re looking at a human trafficking crime here,” and that Homeland Security and Immigration and Customs Enforcement officials were helping in the investigation.
No further details were available about how long the truck had been in the Walmart parking lot, which is on the southwestern side of the city, or where it had come from. Chief McManus said that surveillance video showed that several vehicles had approached the trailer to pick up people.
Chief Hood said the air-conditioning in the truck was not working, adding that those found were “very hot to the touch.”
Of the survivors, he said, “our paramedics and firefighters found that each one of them had heart rates over about 130 beats per minute.”
The truck may have held others. Some occupants fled into the woods nearby, and the police chief said officers would search on foot and by air.
A spokesman for Customs and Border Protection, the parent agency of the Border Patrol, said on Sunday that the people in the truck were probably migrants who had crossed the border with Mexico on foot and taken to a stash house before being put into the tractor-trailer to be transported farther north.
The grisly discovery on Sunday came 14 years after one of the worst episodes of mass migrant death in Texas. In May 2003, 19 undocumented immigrants suffocated and died in the trailer of a milk truck that was found abandoned near Victoria.
Dozens of immigrants, crammed inside the trailer, struggled to survive temperatures as high as 173 degrees as the truck traveled along South Texas highways. Those inside clawed at the truck’s insulation and broke out a taillight in an attempt to get air and alert motorists.
Experts were at odds over whether President Trump’s crackdown on immigration had increased the likelihood of such cases, but Mr. Rodríguez said the 2003 episode illustrated the persistence of the problem.
“We don’t have any good way of measuring if it’s increasing because of Trump, but we know it’s a constant,” he said. “Smuggling is a billion-dollar industry when you look at the whole border.”
Guadalupe Correa-Cabrera, an expert in border issues and a fellow at the Wilson Center, a Washington research institute, said that these types of smuggling services were in greater demand because of the difficulty of crossing the border by other means.
“Events like this are an unintended consequence of enhanced border enforcement and security measures,” she said. “Further enhancing border security puts migrants under greater risk and strengthens transnational human smuggling networks.”
Often on the border in South Texas, migrants cross into the country in small groups on foot but do not travel north immediately. Instead, smugglers organize them into larger groups in stash houses, often in cramped and violent conditions. Those houses are in cities and towns between the border and a network of Border Patrol traffic checkpoints.
Smugglers then transport migrants from the stash houses in large groups in tractor-trailers, or disperse them in smaller vehicles, taking them to cities such as Houston or San Antonio.
Tractor-trailers loaded with migrants that try to make it past the traffic checkpoints without their cargo being detected pose a host of problems: Drivers who turn off the cooling system as they pass the checkpoints may forget to turn it back on, or the cooling system may break down or be ineffective in keeping the migrants cool.
In the Victoria case, the truck’s driver told a Border Patrol agent at the checkpoint that the vehicle was empty and that he was headed to Houston to pick up produce, according to court documents. The agent allowed the driver through without an inspection because the trailer’s refrigeration unit was turned off.
Other cases similar to the one on Sunday have occurred in recent months.
In April, George L. Stewart of Wheeler, Mich., was sentenced to nearly six years in prison after being stopped in Freer, Tex., at a Border Patrol checkpoint. He was trying to smuggle 10 people into the United States in the back of a Penske rental truck.
This month in Houston, about a dozen immigrants being smuggled in a cargo truck were rescued after being left in the locked vehicle for roughly 12 hours in a strip-mall parking lot. A police officer heard the immigrants, including a 16-year-old girl, banging on the walls.
Tom Berg, the first assistant district attorney in Harris County, told reporters at the time, “Thirty more minutes, and this could have been a dozen homicide cases.”