Bradley, of Clearwater, Florida, told authorities he was unaware the immigrants were in the tractor-trailer and he was delivering the truck to a new owner, according to a statement included in the complaint. Another eight people in a less severe condition were taken to another hospital, he said.
The employee brought water to the truck in the parking lot, and then called the police who "found eight people dead in the back of that trailer", the police chief said, calling it a "horrific tragedy".
The gruesome discovery came to light on Saturday when a man from the tractor-trailer approached a Walmart employee "asking for water".More news: Kristen Stewart Crashed This Couple's Wedding And Everyone Loved It
It was over 100 degree Fahrenheit (37.8 degree Celsius) in San Antonio on Sunday.
The driver of the truck, who police did not identify, has been taken into custody.
Police said while reviewing the store's surveillance video they noticed a number of cars had arrived at the store and left with some of survivors.
They did not say whether the trailer was locked when they arrived, whether the rig was used to smuggle the occupants across the border into the U.S., or where it might have been headed.More news: Aboriginal musician Dr G. Yunupingu dies aged 46
Bradley has denied knowing of the contents in his truck, and said he only became aware that he was transporting the victims after he exited to use the restroom.
San Antonio Police Chief William McManus calls the deaths a "human-trafficking crime".
A truck driver has been arrested after dozens of undocumented immigrants were found in the back of his vehicle.
Investigation has been initiated by the city police department. "It was a mass casualty situation for us".More news: Dhawan half-century steers India to 115-1 at lunch
In three previous incidents this month in San Antonio area, federal agents found 72 people, 33 people and 16 people from Mexico, Ecuador, Guatemala and El Salvador inside locked trailers.
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