The man charged with driving a Texas tractor-trailer packed with immigrants in the U.S. illegally will remain in detention after an initial court appearance.

James Matthew Bradley Jr. was handcuffed and wearing blue jail scrubs as U.S. Magistrate Judge Elizabeth Chestney explained he could face the death penalty if convicted. Bradley is charged with transporting immigrants here illegally, resulting in the deaths of 10 people.

Bradley, 60, of Clearwater, Florida didn't speak about what happened Sunday.

Chestney scheduled another hearing Thursday.

A federal complaint says Bradley told authorities he was driving the trailer to Brownsville, Texas, and was unaware that it was packed with people until he stopped at a Wal-Mart in San Antonio.

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James Mathew Bradley Jr., 60, of Clearwater, Florida, arrived in San Antonio, Texas court on Monday in handcuffs

During his court appearance, he didn't speak as he was charged with transporting immigrants to the U.S. illegally, resulting in the deaths of 10

Bradley told authorities he was driving the trailer to Brownsville and was unaware that it was packed with people until he stopped at a Wal-Mart

Authorities fear the death toll could rise because many of those rescued from the sweltering truck in San Antonio have been hospitalized with extreme dehydration and heatstroke. Eight were dead at the scene, while two later died at the hospital.

The federal complaint says the immigrants packed into the tractor-trailer discovered outside a Texas Wal-Mart were taking turns breathing through a hole in the trailer and pounding on the walls to get the driver's attention.

A passenger in the trailer told investigators that he and others who had crossed the U.S.-Mexico border illegally were guided into the trailer to be taken north to San Antonio.

The complaint says passengers appeared fine during the first hour of their journey, but people later began to struggle to breathe. They were trying to get the driver's attention, but to no avail.

Bradley, however, told cops that he was taking the tractor-trailer to its new owner in Brownsville, and had no idea what was in the cargo hold. He said that it wasn't until he made a stop at the Wal-Mart to urinate, that he heard the people pounding on the inside to get out.

'Bradley said he went to open the doors and was surprised when he was run over by "Spanish" people and knocked to the ground,' the affidavit says.

Dan Patrick (pictured), lieutenant governor of Texas, on Sunday blamed the deaths of ten illegal immigrants in San Antonio on sanctuary cities

Bradley told police that he noticed 'bodies just lying on the floor like meat' and that he knew at least one of them was dead. He also said that he knew the trailer's refrigeration system didn't work and that the four vent holes were likely clogged.

'Bradley stated he went back to the tractor and called his wife but she didn't answer. Bradley said he did not call 911,' the affidavit says.

It was a Wal-Mart employee who called to alert 911 about the suspicious vehicle, around 12:30am on Sunday.

When officers arrived on the scene, they found Bradley in the camper, between the cab and the trailer.

It was not immediately known whether Bradley had an attorney who could speak on his behalf.

Meanwhile, the lieutenant governor of Texas has blamed the deaths of ten illegal immigrants on sanctuary cities.

In a Facebook post on Sunday, Dan Patrick said if it wasn't for sanctuary cities, which refuse to work with the federal government in deporting illegal immigrants, the tragedy in San Antonio would never have happened.

'Sanctuary cities entice people to believe they can come to America and Texas and live outside the law. Sanctuary cities also enable human smugglers and cartels.

'Today, these people paid a terrible price and demonstrate why we need a secure border and legal immigration reform so we can control who enters our country. We continue to pray for the families and friends of the victims,' Patrick, a Republican, wrote.

On Sunday morning, emergency crews pulled dozens of people from a sweltering tractor-trailer parked outside a Walmart in the midsummer heat.

Ten people are dead and the death toll could rise, as 30 others were taken fro the rig and hospitalized in dire condition, with extreme dehydration and heatstroke.

Above, Dan Patrick's Facebook post on Sunday about the tragedy

Ten people died after being crammed in the back of an 18-wheeler in San Antonio, Texas this weekend. Above, investigators taking evidence from the scene on Sunday

The ten people were all illegal immigrants. The air conditioning was not working in the back of the truck on a day when the temperatures remained in the 90s

The group were found in the back of the truck, which was parked at a San Antonio Walmart

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.

Authorities would not say whether the trailer was locked when they arrived, but they said it had no working air conditioning.

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

Based on initial interviews with survivors of the San Antonio tragedy, more than 100 people may have been packed into the back of the 18-wheeler at one point in its journey, ICE acting Director Thomas Homan said. Officials said 39 people were inside when rescuers arrived, and the rest were believed to have escaped or hitched rides to their next destination.

Four of the survivors appeared to be between 10 and 17 years old, Homan said. Investigators gave no details on where the rig began its journey or where it was headed.

But Homan said it was unlikely the truck was used to carry the immigrants across the border into the United States. He said people from Latin America who rely on smuggling networks typically cross the border on foot and are then picked up by a driver.

'Even though they have the driver in custody, I can guarantee you there's going to be many more people we're looking for to prosecute,' Homan said.

San Antonio is about a 150-mile drive from the Mexican border. The temperature in San Antonio reached 101 degrees on Saturday and didn't dip below 90 degrees until after 10pm.

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 Sunday, July 23, 2017, in San Antonio

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

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, McManus said.

On Sunday evening, about 100 people gathered at a San Antonio church for a vigil to mourn those killed.

Immigrants' rights activists and church officials held up handmade signs reading 'Who here is not an immigrant' and 'No human is legal.'

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.