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 over the hellish journey.

In outlining their immigrant-smuggling case against James Matthew Bradley Jr., 60, federal prosecutors depicted the trailer as pitch-black, crammed with 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.

He did not enter a plea or say anything about what happened. But in court papers, he told authorities he didn't realize anyone was inside his rig until he parked and got out to relieve himself.

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. Nearly 20 others rescued from the rig were hospitalized in dire condition, many suffering from extreme dehydration and heatstroke.

At least some of those aboard were from Mexico and Guatemala, authorities said.

Bradley told investigators that the trailer had been sold and he was transporting it for his boss from Iowa to Brownsville, Texas. 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 criminal complaint.

Bradley 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 knew at least one passenger was dead.

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

"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. He said he had no idea why Bradley took the looping route he described to investigators.

Bradley told authorities that he had stopped in Laredo, Texas - which would have been out of his way if he were traveling directly to Brownsville - to get the truck washed and detailed before heading back 150 miles (240 kilometers) north to San Antonio. From there, he would have had to drive 275 miles south again to get to Brownsville.

"I just can't believe it. I'm stunned, shocked. He is too good a person to do anything like this," said Bradley's fiancee, Darnisha Rose of Louisville, Kentucky. "He helps people, he doesn't hurt people."

She said Bradley told her he had no idea how the immigrants got into his trailer.

One of the passengers told investigators that he was in a group of 24 people who had been in a "stash house" in Laredo for 11 days before being taken to the tractor-trailer.

A passenger told investigators that he and others crossed into the U.S. by raft, paying smugglers 12,500 Mexican pesos (about $700). They were then put aboard the truck to be taken to San Antonio, according to the criminal complaint. He said he was supposed to pay the smugglers $5,500 once he got there.

He said that those in the truck were given no water or food and that the immigrants soon began to struggle to breathe and started passing out.

Latin Americans who enter the United States illegally often hire smuggling networks to guide them across the border by foot or raft, put them in safe houses and then give them rides to their destination in the U.S.

"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 tragedy came to light when a person from the truck approached a Walmart employee and asked for water late Saturday night or early Sunday morning after a day of temperatures in the 90s and over 100, police said. The employee gave the person water and then called police.

In one of the worst smuggling-by-truck tragedies on record in the U.S., 19 immigrants locked inside a stifling rig died in Victoria, Texas, in 2003. The driver was sentenced to nearly 34 years in prison.

Activists blamed federal and Texas authorities' hard-line immigration policies for contributing to the latest deaths by forcing immigrants 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.

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Associated Press writers Claire Galofaro in Louisville, Ky.; Ryan Foley in Iowa City, Iowa; Scott McFetridge in Schaller, Iowa; Mike Graczyk in Houston; Elliot Spagat in San Diego; Peter Orsi in Mexico City; and Frank Bajak in San Antonio contributed to this report.