In what officials are describing as a possible human trafficking crime, authorities said a Walmart employee called the police after a person who appeared to be distressed asked for water, KENS5 reported.
Authorities discovered the grisly scene after they inspected Bradley's truck in a Walmart parking lot. A total of 39 people were found inside, including two 15-year-old children.
McManus said when SAPD arrived on the scene, they found eight victims dead in the trailer.
Several other people have been taken to hospitals.
"All were victims of ruthless human smugglers indifferent to the well-being of their fragile cargo", US Attorney Richard L. Durbin Jr. said in a statement.
According to court documents, Bradley claims he didn't know anyone was in the truck, telling agents he was surprised when he was "run over by Spanish people and knocked to the ground".
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Mexican officials were working with United States authorities to identify the dead, the country's foreign ministry said.
Based on initial interviews with survivors, Homan said there may have been more than 100 people in the truck. Many suffered from dehydration and heat stroke.
Temperatures in San Antonio had soared to a high of 100 degrees on Saturday, according to the Weather Channel.
He said some of the people who had been in the truck had left to the nearby woods. Authorities would not say whether the trailer was locked when they arrived, but they said it had no working air conditioning.
The Department of Homeland Security is assisting in the investigation.