At least nine people died after being crammed into the back of a sweltering tractor-trailer found parked outside a Walmart in the midsummer Texas heat, authorities said yesterday in what they described as an immigrant-smuggling attempt gone wrong.

The driver was arrested, and nearly 20 others rescued from the rig were hospitalised in dire condition, many with extreme dehydration and heatstroke, officials said.

Authorities were called to the parking lot late Saturday night or early Sunday and found eight dead inside the 18-wheeler. Two more victims died at the hospital, Thomas Homan, acting director of US Immigration and Customs Enforcement, told The Associated Press.

Police Chief William McManus and Fire Chief Charles Hood briefing on tractor trailer found with multiple deceased after being smuggled inside. 8538 IH35S

Posted by San Antonio Police Department on Sunday, 23 July 2017

Based on initial interviews with survivors, Homan said there may have been more than 100 people in the truck at one point. Thirty-eight were found inside, and the rest were believed to have fled or been picked up, authorities said.

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Homan said some of the survivors told authorities they were from Mexico.

It was just the latest smuggling-by-truck operation to end in tragedy. In one of the worst cases on record in the US, 19 immigrants locked inside a stifling rig died in Victoria, Texas, in 2003.

"We're looking at a human-trafficking crime," Police Chief William McManus said, adding that many of those inside the 18-wheeler appeared to be in their 20s and 30s and that there were also apparently two school-age children.

He called it "a horrific tragedy."

Authorities did not say whether the rig was locked when they arrived, whether it was used to smuggle the occupants across the border into the US, or where it might have been headed. San Antonio is about a 240-kilometre drive from the Mexican border.

There was no immediate word on any charges brought against the driver, whose name was not released. The US Homeland Security Department stepped in to take the lead in the investigation.

Crime scene tape is removed from around a Walmart in San Antonio, Texas, after the discovery of a tractor-trailer unit with eight people dead inside. Photo / AP
Crime scene tape is removed from around a Walmart in San Antonio, Texas, after the discovery of a tractor-trailer unit with eight people dead inside. Photo / AP

The victims "were very hot to the touch. So these people were in this trailer without any signs of any type of water," San Antonio Fire Chief Charles Hood said.

The temperature in San Antonio reached 38C on Saturday and didn't dip below 32C until after 10pm. The trailer didn't have a working air conditioning system, Hood said.

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The tragedy came to light after a person from the truck approached a Walmart employee in the parking lot and asked for water late Saturday night or early Sunday morning, McManus said.

The employee gave the person water and then called police, who found the dead and the desperate inside the rig. Some of those in the truck ran into the woods, leading to a search, McManus said.

Hours later, after daybreak, a helicopter hovered over the area, and investigators were still gathering evidence from the tractor-trailer, which had an Iowa license plate and was registered to Pyle Transportation Inc. of Schaller, Iowa. A company official did not immediately respond to a phone message seeking comment.

Investigators checked store surveillance video, which showed vehicles arriving and picking up people from the truck, authorities said."

San Antonio police officers investigate the scene where eight people were found dead in a tractor-trailer. Another two have since died. Photo / AP
San Antonio police officers investigate the scene where eight people were found dead in a tractor-trailer. Another two have since died. Photo / AP

By any standard, the horrific crime uncovered last night ranks as a stark reminder of why human smuggling networks must be pursued, caught and punished," Homan said in a statement.

In the May 2003 case, the immigrants were being taken from South Texas to Houston.

Prosecutors said the driver heard them begging and screaming for their lives but refused to free them. The driver was sentenced to nearly 34 years in prison.

The Border Patrol has reported at least four truck seizures this month in and around Laredo, Texas. On July 7, agents found 72 people crammed into a truck with no means of escape, the agency said. They were from Mexico, Ecuador, Guatemala and El Salvador.

Authorities in Mexico have also made a number of such discoveries over the years.

Last December, they found 110 migrants trapped and suffocating inside a truck after it crashed while speeding in the state of Veracruz. Most were from Central America, and 48 were minors. Some were injured in the crash.

Last October, also in Veracruz state, four migrants suffocated in a truck carrying 55 people.

- AP