The authorities are treating it as a human trafficking incident.
Twenty people were airlifted to seven hospitals in conditions ranging from "critical to very critical", Hood said. San Antonio Fire Chief Charles Hood told reporters that there were 38 people in the back of the tractor trailer, including two as young as 15 years old, and described the passengers as "very hot to the touch".
In Texas alone, federal immigration agents said they arrested 123 illegal immigrants with criminal records in an eight-day operation that ended last week.
San Antonio Police Chief William McManus briefed media at the scene, saying the driver of the truck had been arrested.
Surveillance footage showed that several vehicles had entered Walmart's parking lot before police were alerted to the case and picked up "lots of folks that were in that trailer that survived the trip", McManus said.
Walmart employees become aware of the situation after one of the aliens approached them and asked for water.
"We're looking at a human trafficking crime this evening", he added.
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Harris County District Attorney's Office First Assistant Tom Berg said, "Thirty more minutes and this could have been a dozen homicide cases".
The employee "came back with the water, called the police, and we found eight dead in the back of that trailer", McManus said.
Once the victims undergo treatment, the case will be turned over to US Immigration and Customs Enforcement, the police chief said. They were stuffed in a trailer with more than a hundred people in it near Victoria, southeast of San Antonio.
Authorities have not said whether the rig was locked when cops arrived, or whether it was used to smuggle the almost 40 people across the border into the US. Thirty-eight people were in the trailer.
As a helicopter hovered over the area Sunday morning, investigators were still sifting the scene for evidence. McManus stressed that it was not an isolated incident.
The driver of the truck has been taken into custody. The eight adult men who were found dead are believed to have been killed by heat exposure or asphyxiation, according to police. "If they were to spend another night in that environment, you'd have 38 people who would not have survived". Nineteen of the immigrants died from dehydration, overheating and suffocation during the smuggling attempt from South Texas to Houston. "This happens quite frequently under cover of night".