The driver has been arrested after eight people were found dead in a tractor trailer outside a Walmart store in what police called a horrific human trafficking case.

Several other people, possibly dozens, were found in the truck, which did not have a working air conditioning system despite blistering temperatures, and were taken to hospitals.

The driver had been arrested but his identity was not released.

A person from the truck approached a Walmart employee in a car park and asked for water late on Saturday night or early Sunday morning, said police in San Antonio, where temperatures on Saturday reached 38C.

The employee gave the person the water and then called police, and when officers arrived found the eight people dead in the back of the trailer, police Chief William McManus said.

Investigators checked store surveillance video, which showed vehicles had arrived and picked up other people from the tractor trailer.

"We're looking at a human trafficking crime this evening," Mr McManus said.

He called the case "a horrific tragedy".

The US Department of Homeland Security was involved in the investigation into what happened.

The National Weather Service's local office said the temperature in San Antonio hit 38C just before 5pm on Saturday and did not dip below 32C until after 10pm.

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