San Antonio Police Chief William McManus and Fire Chief Charles Hood told reporters that in addition to the dead, there were 28 injured -- 20 of them severely -- who were being treated at seven local hospitals.
"We arrived on the scene and found eight people dead in the back of that trailer," the police chief said, calling it horrific.
He explained that store security footage showed that some vehicles came to pick up some travellers who were on the truck and made it alive.
"We're looking at a human trafficking crime this evening," he added.
It was not immediately clear how many people made it off the truck alive, McManus said.
San Antonio lies a few hours drive from the border with Mexico's Nuevo Leon state. Weather in the area has been hot and dry.
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