Nine people have been found dead and more than 30 seriously injured in the back of a truck outside a Walmart store in San Antonio, Texas, in what police are calling a “horrific” human trafficking case.

City officials said on Sunday [US time] that 39 people had been in the truck and 30 were taken to hospitals, 20 of whom about 20 were in “extremely severe” or critical condition.

“We got a call from a Walmart employee about a welfare check in a tractor-trailer that was parked on the lot here,” San Antonio police chief William McManus said.

“He was approached by someone from that truck, who was asking for water.

“We’re looking at a human trafficking crime this evening.”

Officials said many of those inside the truck were adults aged in the 20s to 30s as well as what appeared to be two school-age children.

The truck’s driver has since been arrested.

“These people were helpless in the hands of their transporters,” US Attorney for the Western District of Texas Richard Durbin said in a statement.

He said heat in southern Texas is “punishing” at this time of year, with temperatures around 38 degrees Celsius.

“Imagine their suffering, trapped in a stifling trailer in 100-plus degree (Fahrenheit) heat,” he said.

The temperature in San Antonio reached 38degrees Celsius on Saturday and did not go below 32 degrees Celsius until after 10pm.

They were victims of “ruthless human smugglers indifferent to the wellbeing of their fragile cargo,” he added.

It was not made immediately clear how long the truck had been in the parking lot.

“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,” acting Immigration and Customs Enforcement director Thomas Homan said in a statement.

“These networks have repeatedly shown a reckless disregard for those they smuggle, as last night’s case demonstrates.”

The latest in a string of seizures

Other cases of human trafficking in the United States have led to more deaths.

In May 2003, 19 immigrants being transported from South Texas to Houston died inside a sweltering truck.

Prosecutors said the driver in the 2003 case heard the immigrants begging and screaming for their lives as they were succumbing to the stifling heat inside his truck but refused to free them.

The driver was resentenced in 2011 to nearly 34 years in prison after a federal appeals court overturned the multiple life sentences he had received.

The Border Patrol has reported at least four truck seizures this month in and around Laredo, Texas.

On July 7, agents found 72 people from Mexico, Ecuador, Guatemala and El Salvador crammed into a truck with no means of escape, the agency said.

– with ABC