SAN ANTONIO (AP) — The driver of a broiling tractor-trailer found packed with immigrants outside a Walmart in San Antonio was charged Monday in the deaths of 10 of his passengers and could face the death penalty.

In outlining their immigrant-smuggling case against James Matthew Bradley Jr., 60, federal prosecutors depicted the trailer as pitch-black, crammed with somewhere around 90 people or more by some estimates, and so suffocatingly hot that one passenger said they took turns breathing through a hole and pounding on the walls to get the driver’s attention.

Bradley appeared in federal court on charges of illegally transporting immigrants for financial gain, resulting in death. The Clearwater, Florida, man was ordered held for another hearing on Thursday and said nothing in court about what happened.

Over the weekend, authorities discovered eight bodies inside the crowded 18-wheeler parked in the summer heat, and two more victims died at the hospital. Officials feared the death toll could rise because nearly 20 others rescued from the truck were in dire condition, many suffering from extreme dehydration and heatstroke.

Bradley told investigators that the trailer had been sold and he was transporting it for his boss from Iowa to Brownsville, Texas, and that he was unaware people were inside until he parked and got out to urinate.

After hearing banging and shaking, he opened the door and was “surprised when he was run over by ‘Spanish’ people and knocked to the ground,” according to the complaint.

Bradley allegedly told investigators that he knew the trailer refrigeration system didn’t work and that the four ventilation holes were probably clogged. He also said he did not call 911, even though he realized several people already were dead.

The truck had an Iowa license plate and was registered to Pyle Transportation Inc. of Schaller, Iowa. Company President Brian Pyle said that he had sold the truck to a man in Mexico in May and that Bradley was an independent contractor who was supposed to deliver it to a pick-up point in Brownsville.

Pyle said he had no idea of any problems with the truck until the news media started to call Sunday following reports of the deaths in San Antonio.

“I’m absolutely sorry it happened. I really am. It’s shocking. I’m sorry my name was on it,” Pyle said, referring to the truck.

A passenger told investigators that he and others crossed the Mexican border into the U.S. by raft and were then guided into the trailer to be taken to San Antonio, which is about a 150-mile (240-kilometer) drive from the Mexican border, according to the criminal complaint. He said those in the truck were given no water or food.

The passenger said the immigrants inside the truck soon began to struggle to breathe and started passing out.

At least some of those in the truck were from Mexico and Guatemala, according to diplomats from the two countries.

Latin Americans who enter the U.S. illegally often rely on smuggling networks to guide them across the border, supply them with safe houses and give them rides in the U.S. to their destination.

“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,” said Thomas Homan, acting director of U.S. Immigration and Customs Enforcement.

The temperature in San Antonio reached 101 degrees (38 Celsius) on Saturday and didn’t dip below 90 degrees (32 C) until after 10 p.m.

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 police chief. The employee gave the person water and then called police.

It was 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.

Immigrants’ rights activists have blamed federal and Texas authorities’ hard-line immigration policies for contributing to the deaths by forcing migrants to take greater risks to reach the U.S.

“These tragedies are compounded when it’s incredibly dangerous and incredibly expensive and we push migration into the hands of illicit actors,” immigration activist Bob Libal said.

In the May 2003 case, the immigrants were being taken from South Texas to Houston. Prosecutors said the driver heard them begging and screaming for their lives but refused to free them. The driver was sentenced to nearly 34 years in prison.

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.

Authorities in Mexico have also made a number of similar discoveries over the years.

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Associated Press writers Mike Graczyk in Houston, Elliot Spagat in San Diego, Peter Orsi in Mexico City and Frank Bajak in San Antonio contributed to this report.

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This story has been corrected to show that people at the vigil in San Antonio held signs saying “No human is illegal,” not “No human is legal.”