CHICAGO — A survivor of a horror truck journey in which 10 migrants suffocated to death has told how travellers took turns breathing through a tiny hole in a desperate bid to stay alive, US investigators said on Monday.

As charges were filed against the driver who was detained in Texas near the border with Mexico, one of President Donald Trump’s cabinet secretaries denounced the "brutality" of people-smuggling gangs.

And two children were among a group of more than two dozen people still in hospital, suffering from heat stroke and dehydration, after an ordeal which ended in a parking lot.

The migrants were discovered in the back of the 18-wheel truck in the early hours of Sunday in San Antonio, Texas, a two-hour drive from the US-Mexico border, when one of them approached a Walmart store employee asking for water.

The employee brought water and then called police, who found 38 people crammed in the trailer with a broken refrigeration system, parked in the baking Texas heat.

Eight people were pronounced dead at the scene and two others died later at the hospital.

The sweltering trailer may have held between 70 to 200 people, with some migrants fleeing in six SUVs that had been waiting when the truck stopped in the parking lot, according to witness accounts given to authorities.

’Driver never stopped’

The document recounted a harrowing journey, with migrants having trouble breathing and some passing out in the trailer which was being driven by James Mathew Bradley Jr, age 60.

"People began hitting the trailer walls and making noise to get the driver’s attention. The driver never stopped," according to one of the migrants interviewed, identified only as J.M.M-J.

"People had a hole in the trailer wall to provide some ventilation and they started taking turns breathing from the hole."

J.M.M-J said he was a Mexican national and part of a group of 29 people being smuggled into the US. He said that after crossing the border they joined 70 migrants already in the truck’s trailer.

US authorities have not released all the victims’ nationalities or names, pending notification of their families.

Mexico’s foreign ministry said four of the dead were Mexican, out of a total of 25 Mexicans riding on the truck. The other 21 remain hospitalised.

The Guatemalan foreign ministry said 20-year-old Frank Fuentes was among those killed, and his family had already been contacted to begin the process of repatriating his body.

Two other Guatemalans, one of them a minor, were hospitalised and in stable condition, the ministry said.

According to the migrant’s recollection, his smuggler said "people linked to the Zetas" crime cartel were offering protection for the journey through Mexico to the US border, and that once arriving in the country he was to pay $5,500.

The driver was formally charged on Monday with one count of transporting illegal aliens.

The federal charge against Bradley is punishable by life imprisonment or even theoretically the death penalty.

Bradley has told police he did not know he was transporting people until he stopped at the Walmart store to use the restroom and observed "banging and shaking," prosecutors said.

’Network of death’

Bradley said he was delivering the trailer from Iowa to Texas on the orders of his boss, and attempted to administer aid when he found the migrants, but did not call 911, according to the complaint.

Thirty people were hospitalised -- including two school-age children -- and more than a dozen were in critical condition, suffering from heat stroke and dehydration, authorities said. It was unclear how long the migrants were inside the truck.

Homeland Security Secretary John Kelly in a statement called the deaths "senseless" and the result of a human trafficking "network of abuse and death".

"This tragedy demonstrates the brutality of the network of which I often.speak. These smugglers have no regard for human life and seek only profits," Kelly said.

Kelly has been to Mexico twice to discuss immigration, human trafficking and the sprawling cross-border drug trade. — AFP