A person from the lorry initially approached a Walmart supermarket employee in the parking lot and asked for water late on Saturday night or early on Sunday morning, police Chief William McManus said. A total of 39 people were found inside, including two 15-year-old children.
Eight passengers were found dead when the San Antonio Police Department arrived and around 20 needed immediate medical care.
"All were victims of ruthless human smugglers indifferent to the well-being of their fragile cargo", San Antonio-based US Attorney Richard Durbin Jr said.
Authorities don't know how long that truck had been sitting in the Walmart parking lot or where it came from, but they know it had no air conditioning.
The driver of the lorry found in the auto park at the store in San Antonio was arrested and taken into police custody, although his identity has not yet been released.
Between 30-40 others ran from the truck trailer when he opened the doors upon hearing banging and screaming within. Apart from recovering the eight dead bodies, the officials also rescued 23 people from the tractor-trailer, who were in critical condition.
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The employee handed over the water but quickly called the police, who immediately arrested the driver of the truck . Breaking 2:28am - SAFD confirms multiple bodies found inside a trailer parked outside a Walmart in SW San Antonio .
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Dewey, 64, said people in their 30s have opened several new businesses and wine bars, attracting lots of young tourists. Officials said the fire was 15 percent contained after almost $11 million was spent to battle the blaze.
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Video footage from the store showed a number of vehicles arriving to pick up some of the survivors, he added.
The U.S. Homeland Security Department is taking the lead in the investigation.
Those still alive are expected to have incurred brain damage from the severe heat.
On Sunday the temperatures hit 100 degrees in San Antonio during the day.
At least nine people have died after being crammed into the back of a tractor-trailer and traveling under scorching conditions, officials say, in an update on a case of apparent human trafficking. He added the driver is likely to face both state and federal charges, WCVB5 reported. "Fortunately, there are people who survived".