Eight people were found dead in a tractor-trailer outside a Walmart store in San Antonio in the stifling summer heat. The police called the incident "a horrific human trafficking crime".

The driver has been arrested, but his identity has not been disclosed.

Several other people, possibly dozens, were found in the truck, which didn't have a working air conditioning system despite blistering temperatures that topped 100 degrees Fahrenheit (38 degree Centigrade), and were taken to hospitals, said authorities.

According to police in San Antonio, a person from the truck approached a Walmart employee in a parking lot and asked for water late Saturday night.

After giving water to the person, the employee then called police. When officers arrived, they found eight people dead in the back of the trailer, said police Chief William McManus.

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

“We're looking at a human trafficking crime this evening,” McManus said.

He called the case “a horrific tragedy”. The US Department of Homeland Security was involved in the investigation of the incident, he said.

The National Weather Service's local office said that the temperature in San Antonio hit 101 degrees Fahrenheit just before 5pm on Saturday and didn't dip below 90 degrees Fahrenheit (32 degrees Centigrade) until after 10pm.