By news.com.au's Debra Killalea and AP

From the outside, nothing seemed amiss: A nondescript truck parked outside a shopping centre in the 38C heat of a northern hemisphere summer.

Inside, though, it was hell on wheels.

As the truck sat on the searing tarmac, the truck's tragic human cargo began cooking to death in unbearable, 50C temperatures. Thirsty, sweating and with their hearts racing, some lost consciousness. Eight would die there on the spot.

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Evidence is collected at the scene where nine people were found dead in a tractor-trailer loaded with multiple others, outside a Walmart store. Photo / AP
Evidence is collected at the scene where nine people were found dead in a tractor-trailer loaded with multiple others, outside a Walmart store. Photo / AP

No one knows how long the truck - with no working air conditioning - had sat there. It was only when a Walmart security guard was asked for water in the carpark that suspicions were finally aroused.

But by the time authorities arrived at the shopping centre, in San Antonio, Texas, it was already far too late.

Experts now say some of the 30 others taken to hospital may never recover from irreversible brain damage suffered as a result of the suffocating conditions.

A ninth person has since died.

Described as "an alien-smuggling venture gone horribly wrong", the group were likely part of a wider human trafficking tragedy and the latest to hit the state of Texas.

The exact nationalities of those inside as well as their ages remains unconfirmed, however some are understood to be Mexican. All nine dead were men.

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

Thomas Homan, acting director of US Immigration and Customs Enforcement said 38 people were found inside but authorities believed as many as 100 were initially crammed into the vehicle. The rest are believed to have fled or been picked up.

It remained unclear whether the truck was used to smuggle the occupants across the border into the US, or where it was headed.

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San Antonio is about 240km from the Mexican border.

THE TIP OFF

According to San Antonio Fire Department spokesman Joe Arrington, a Walmart security guard, was approached and alerted to the fact the trailer was full of immigrants.

San Antonio Police Chief William McManus said the informant, who had been in the truck, had approached the guard looking for water, according to NBC News.

San Antonio police officers investigate the scene where nine people were found dead in a tractor-trailer loaded with at least 30 others outside a Walmart store. Photo / AP
San Antonio police officers investigate the scene where nine people were found dead in a tractor-trailer loaded with at least 30 others outside a Walmart store. Photo / AP

The security guard inspected the truck and found the group inside.

'IRREVERSIBLE BRAIN DAMAGE'

San Antonio Fire Chief Charles Hood said the trailer didn't have a working air conditioning system, despite being a refrigerated truck.

Hood said the paramedics and firefighters who treated the victims found all had accelerated heartbeats with no water on hand.

"We are very fortunate that there weren't 38 people who were locked inside of this vehicle dead", Hood said.

"Our paramedics and firefighters found that each patient had heart rates over about 130 beats per minute and were very hot to the touch."

San Antonio police officers investigate the scene where nine people were found dead in a tractor-trailer loaded with at least 30 others outside a Walmart store. Photo / AP
San Antonio police officers investigate the scene where nine people were found dead in a tractor-trailer loaded with at least 30 others outside a Walmart store. Photo / AP

The victims died as a result of heat exposure and suffocation, according to a San Antonio Police Department statement.

However the Bexar County Medical Examiner's Office will determine the official cause of death.

The fire chief described how he quickly called a "mass casualty incident", CNN reported.

"With heat strokes or heat injuries, a lot of them are going to have some irreversible brain damage," he said.

A San Antonio police officer removes crime scene tape from near the area where nine people were found dead in a tractor-trailer loaded with multiple others, outside a Walmart store. Photo / AP
A San Antonio police officer removes crime scene tape from near the area where nine people were found dead in a tractor-trailer loaded with multiple others, outside a Walmart store. Photo / AP

In a statement Texas Governor Greg Abbott said the loss of lives in the suspected human trafficking incident was a heartbreaking tragedy.

"Human trafficking is an epidemic that Texas is working to eradicate," he said.

"To that end, Texas will continue to provide protection for the victims who have been robbed of their most basic rights, and bring down the full weight of the law for the perpetrators of this despicable crime."

DRIVER HELD

Police checked store surveillance video, which showed vehicles had arrived and picked up other people from the semi-trailer.

Charges are expected to be filed against a Florida man in connection to the deaths.

James Mathew Bradley Jr, 60, of Clearwater, Florida, is in federal custody in San Antonio, US Attorney Richard Durbin Jr said today.

Bradley is expected to make a court appearance after a criminal complaint is filed, but his office wouldn't say whether he was the alleged driver of the truck.

He described those responsible for the deaths as "ruthless human smugglers indifferent to the well-being of their fragile cargo".

San Antonio police officers investigate the scene where nine people were found dead in a tractor-trailer loaded with at least 30 others outside a Walmart store. Photo / AP
San Antonio police officers investigate the scene where nine people were found dead in a tractor-trailer loaded with at least 30 others outside a Walmart store. Photo / AP

Durbin said those inside the truck were helpless in the punishing 38C heat and were victims of an "alien-smuggling venture gone horribly wrong".

The truck had a logo of Pyle Transportation on it.

Mike Pyle and Tom Colton from Pyle Transportation told My San Antonio the trailer was theirs, but the truck itself belonged to Bradley.

The company said he was not authorised to be hauling anything in Texas at the time the tragedy was discovered.

McManus told CNN police were investigating a human trafficking crime.

"Checking the video from the store, we found there were a number of vehicles that came in and picked up a lot of the folks that were in that trailer that survived the trip," he said.

TRAGIC HISTORY

The tragedy isn't the first involving immigrants being smuggled across the border.

In May 2003, 19 immigrants being moved from South Texas to Houston died inside a sweltering tractor-trailer in similar conditions.

Prosecutors said the driver heard the immigrants begging and screaming for their lives as they were succumbing to the stifling heat, but he 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.

Eldia Contreras wipes away a tear as she takes part in a vigil at San Fernando Cathedral for victims who died as a result of being transported in a tractor-trailer. Photo / AP
Eldia Contreras wipes away a tear as she takes part in a vigil at San Fernando Cathedral for victims who died as a result of being transported in a tractor-trailer. Photo / AP

The US Homeland Security Department is taking the lead in the current investigation.

US Border Patrol also reported four other truck seizures this month in and around Laredo, Texas.

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

Last December, Mexican authorities found 110 migrants trapped and suffocating inside a truck after it crashed while speeding in the state of Veracruz.

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