Smugglers are charging desperate migrants $5,500 for the 'VIP treatment' to the US - crammed in the back of big rigs without air conditioning - and women say they prefer it as they are less likely to be raped.

Ten people died after they were locked inside a boiling truck without air conditioning or water on Sunday. Authorities found a total of 38 in the truck, which was parked outside a Texas Walmart, who were all rushed to hospital.

Thomas Homan, the acting director of U.S. Immigration and Customs Enforcement, said he was saddened to hear these kind of tragedies still occurred after the last big human trafficking incident in 2003 when a truck was found 120 miles southeast of San Antonio with 19 dead migrants.

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James Mathew Bradley Jr., 60, of Clearwater, Florida, center, (pictured as he is escorted out of the federal courthouse, Monday) was arrested in connection with the deaths of multiple people packed into a broiling tractor-trailer on Sunday

Ten people died and and 28 were rushed to nearby hospitals in 'critical or very serious condition' after the discovery (Pictured, police officers work on a crime scene after eight people believed to be illegal immigrants were found dead on Sunday morning)

'It is sad that 14 years later people are still being smuggled in tractor-trailers,' he said. 'There still isn't water, there still isn't ventilation. These criminal organizations, they're all about making money.'

Big rigs emerged as a popular smuggling method in the early 1990s amid a surge in U.S. border enforcement in San Diego and El Paso, Texas, which were then the busiest corridors for illegal crossings.

Before that, people paid small fees to mom-and-pop operators to get them across a largely unguarded border. As crossing became exponentially more difficult after the 2001 terror strikes in the U.S., migrants were led through more dangerous terrain and paid thousands of dollars more.

Women, some carrying children, think they are less likely to be raped on a truck than in the open desert because there are more witnesses, Guadalupe Correa-Cabrera, a political scientist who teaches at University of Texas, Rio Grande Valley, said. Riding in a big rig, she said, is 'the VIP treatment.'

Ten people, including two children, were founded dead. Police are investigating the incident as a 'human trafficking' case (Pictured, police officers near a white truck in the parking lot)

It was first discovered that something was amiss when one person approached a Walmart worker in the store parking lot (pictured) and asked for some water

Correa-Cabrera said migrants she interviewed last year in South Texas paid $2,000 to $3,000 more to ride in the crammed tractor-trailers, considering them more effective, faster and safer than walking through the desert to a pickup point far from the border.

Hundreds of border crossers perish each year in the desert, getting lost and dehydrated in extreme heat.

But one of the 38 smuggled immigrants, Mexican laborer Adan Lara Vega, 27, says he was charged $5,500 for the journey. He said that price was supposed to include air conditioning but there was no ventilation and he heard people crying and asking for water before he passed out.

'It's like any other business: the more they move, the more profit they make,' Homan said. 'Rather than taking four in a car, the profit margin on tractor-trailers is a lot more.'

The growing use of trucks coincided with increased trade with Mexico under the North American Free Trade Agreement, allowing smugglers to more easily blend in with cargo, particularly on Interstate 35 from Laredo, Texas, to San Antonio, Correa-Cabrera said. Walking in the open desert more easily exposes them to U.S. Border Patrol agents.

A couple visits a make-shift memorial in the parking lot of a Walmart store near the site where authorities Sunday discovered the tractor-trailer packed with immigrants

Truck drivers are low-level cogs in a big machine, recruited in the U.S. at casinos and other places where smuggling organizations look for people who are down on their luck, desperate for quick cash and disinclined to ask questions.

James Matthew Bradley Jr., who made an initial court appearance Monday in San Antonio on smuggling charges, told authorities he was delivering what he thought was a sold vehicle from Schaller, Iowa, to Brownsville, Texas, and that he didn't know what was inside, according to the complaint. He said he was given no deadline or address to deliver the truck.

Other guides take migrants across Mexico by bus. Others join them on a raft across the Rio Grande or through the desert to a hideout or to a nearby house where they may wait days or weeks. Eventually smuggling organizations get them to major cities like Phoenix, Houston or San Antonio.

'I have to imagine that their winning percentage is really, really high,' said Adam Isacson of the Washington Office on Latin America, a human rights advocacy group. 'Whatever reputation they lose from episodes like this, their profit margins are still high enough to make it work. Otherwise people wouldn't pay.'

The people inside the tractor-trailer were found in the early hours of Sunday morning after one of them approached a Walmart worker in the store parking lot and asked for some water.

James Matthew Bradley Jr., (pictured) who made an initial court appearance Monday in San Antonio on smuggling charges, told authorities he was delivering what he thought was a sold vehicle from Schaller, Iowa, to Brownsville, Texas, and that he didn't know what was inside

The employee gave the person the water and then called police, and when officers arrived they found the eight people - all men - dead in the back of the trailer, police Chief William McManus said.

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

'We're looking at a human trafficking crime this evening,' McManus said.

He called the case 'a horrific tragedy'.

A criminal complaint found a degree of sophistication and organizational muscle: passengers had color-coded tape to split into smaller groups; and six black SUVs awaited them at one transit point to bring them to their destinations.

The police chief added that the Department of Homeland Security has joined an investigation to figure out what happened with the truck.

ICE also released a statement about the incident on Sunday afternoon, saying the deaths are a: 'stark reminder of why human smuggling networks must be pursued, caught and punished'.

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

A Walmart employee said a migrant asked for a glass of water, which he gave them, and then called police, and when officers arrived they found the eight people - all men - dead in the back of the trailer, police Chief McManus said (Pictured, a police officer is seen shining a light into the back of the truck where eight people were found dead)

'The men and women of ICE are proud to stand alongside our law enforcement partners, including locally and at the U.S. Department of Justice, to combat these smuggling networks and protect the public and those who would fall victim to their dangerous practices that focus solely on their illicit profits.'

The city's fire chief, Charles Hood, said rescuers swiftly got patients out of the back of the truck.

He added: 'We had another 20 patients that were either in extremely critical condition or very serious condition and they have been transported to a number of hospitals.

'They were very hot to the touch. So these people were in this trailer without any signs of any type of water.'

Police said Immigration and Customs Enforcement has been notified.

In May 2003, 19 people being transported from South Texas to Houston died inside a sweltering tractor-trailer.

Prosecutors said the driver in the 2003 case heard the people 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.