John Bacon and Sue Calberg, USA TODAY NETWORK

SAN ANTONIO — A truck driver who told police he was unaware of the human cargo packed in his rig was charged Monday with illegally transporting immigrants as the death toll from a weekend smuggling tragedy rose to 10.

James Mathew Bradley Jr., 60, could face the death penalty after eight bodies were discovered just before midnight Saturday in the sweltering trailer of his big rig in a Walmart parking lot. The charges include illegally transporting immigrants for financial gain resulting in death.

One passenger told authorities about 100 people stuffed in the trailer took turns breathing through a hole in the wall, according to the criminal complaint affidavit.

At least 30 people in the truck were rushed to hospitals, many in critical condition. One died Sunday and another Monday, the Department of Justice said. U.S. Immigration and Customs Enforcement is leading the investigation, and acting director Thomas Homan said dozens of people may have fled the scene before authorities arrived.

"To maximize their criminal profits, these human smugglers crammed more than 100 people into a tractor-trailer in the stifling Texas summer heat," Homan said. "Human smugglers have repeatedly demonstrated that they have absolutely no regard for human life."

The affidavit, filed by Homeland Security Investigations Special Agent James Lara, provides a glimpse of Bradley's claim of ignorance as well as the tales of immigrants on an epic journey that ended in horror.

Police were called to the scene by a Walmart employee who told them the trailer appeared to contain multiple people in need of assistance. Officers quickly encountered Bradley, who told them he had driven up from Laredo, about 150 miles away, but didn't know people were packed in the trailer until he stopped to urinate and heard banging from inside.

Bradley said he then opened the truck and was knocked to the ground by "Spanish" people pouring out of the truck, the affidavit said.

"Bradley said he then noticed bodies just lying on the floor like meat," the affidavit said. He told police he attempted to aid the victims but did not call 911, according to the affidavit.

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The unidentified passenger told authorities he was with a group of 28 people who were shuttled across the Rio Grande near Laredo, Texas, in rafts for the equivalent of about 700 U.S. dollars. They then walked through the night before being driven in pickups to the trailer and packed in with about 70 other people.

The plan was to pay the smugglers $5,500 once in San Antonio, he said. He said the trailer was hot but everyone seemed to be alright for about an hour. Then people began passing out.

When Bradley opened the door, those who were able fled to waiting SUVs and were gone before police arrived, the passenger said.

The temperatures in San Antonio on Saturday exceeded 100 degrees, and the truck had no functioning air conditioning, authorities said. All the dead were described as adult men.

Homan said some survivors have identified themselves as Mexican nationals. San Antonio Fire Department Chief Charles Hood reported 17 people were transported to hospitals with life-threatening injuries and another 13 suffered non-life-threatening injuries.

Survivor Adan Lalravega, 27, told the Associated Press that people cried and begged for water and that he heard the whimpering sounds of children's voices. He said he lost consciousness and awoke in a San Antonio hospital bed.

Lalravega said he had been told by smugglers who hid him and six friends in a safe house on the border in Laredo that they would be riding in an air-conditioned space. Instead, he said when they boarded the truck it was dark and already packed with people. He said they were never offered water.

Nicole Perez, who lives behind the Walmart, watched the scene unfold through her back window.

"It was maybe 1:30 and we heard back to back cops, ambulances, fire trucks, helicopter," Perez said. "We came outside and the helicopter was going around in circles."

Norma Ogden, who lives nearby, woke up to the sirens.

"The sirens just kept going and going. It was nonstop. I thought to myself, this is something really bad," Ogden said.

Calberg report for KENS-TV, San Antonio; Bacon reports for USA TODAY in McLean, Va. Contributing: Associated Press