Eight bodies were recovered from parked lorry’s trailer where 28 people also suffered critical injuries.

Eight dead bodies and 20 people in dire condition have been found in the back of a sweltering lorry at a supermarket parking lot in the US city of San Antonio, the state of Texas.

The lorry’s driver was arrested and all 28 survivors were taken to hospitals in what police are calling a horrific case of immigrant smuggling, authorities said on Sunday.

Twenty people were in extremely critical or serious condition and eight others were being treated for lesser injuries, including heat stroke and dehydration.

Temperatures in San Antonio reached 38C on Saturday and did not dip below 32C until after 10pm, according to the National Weather Service.

The lorry’s trailer also did not have a working air conditioner system, San Antonio Fire Chief Charles Hood said in a news briefing.

“They were very hot to the touch. So these people were in this trailer without any signs of any type of water,” he said. “It was a mass casualty situation for us.”

A person from the lorry initially approached a Walmart supermarket employee in the parking lot and asked for water late on Saturday night or early on Sunday morning, police Chief William McManus said.

The employee gave the person the water and then called police, who found the dead and desperate inside the truck.

McManus did not release the arrested driver’s name.

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

“We’re looking at a human trafficking crime this evening,” McManus said, adding many of those inside the vehicle appeared to be adults in their 20s and 30s but that there were also what appears to be two school-age children, as well.

He called the case “a horrific tragedy”.

The US Department of Homeland Security is assisting in the investigation.