The truck's driver was arrested and all 28 survivors were taken to hospitals, where 20 were in extremely critical or serious condition, authorities said. Eight others were being treated for lesser injuries, including heat stroke and dehydration.
Temperatures in San Antonio reached 38 Celsius yesterday and didn't dip below 32 C until after 10 pm, according to the National Weather Service.
"They were very hot to the touch. So these people were in this trailer without any signs of any type of water," he said. "It was a mass casualty situation for us."
A person from the truck initially approached a Walmart employee in the parking lot and asked for water late last night or early this morning, police Chief William McManus said. The employee gave the person the water and then called police, who found the dead and desperate inside the truck.
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Investigators checked store surveillance video, which showed vehicles had arrived and picked up other people from the tractor-trailer, police said.
"We're looking at a human trafficking crime this evening," McManus said, adding many of those inside the truck appeared to be adults in their 20s and 30s but that there were also what appears to be two school-age children, as well. He called the case "a horrific tragedy."
The US Department of Homeland Security is assisting in the investigation.
Other cases of human trafficking in the United States have led to more deaths. In May 2003, 19 immigrants being transported from South Texas to Houston died inside a sweltering tractor-trailer.
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