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

James Mathew Bradley Jr., 60, could face the death penalty on charges filed 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.

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

The criminal complaint says police were called to the seen by a Walmart employee and spoke with Bradley, who told them he didn't know people were packed into the trailer until he stopped to urinated 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 complaint said.

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

One of the immigrants told authorities he was paying $5,500 for a trip across the border, and that about 100 people had been packed into the trailer.

Homan said some survivors have identified themselves as Mexican nationals. Four of the passengers are believed to be between 10 and 17 years old, and at least one of them is in serious condition. All of the victims who died were adult men, authorities said.

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.

The temperatures Saturday exceeded 100 degrees, and the truck had no functioning air conditioning, authorities said. San Antonio Police Chief William McManus said the trailer appeared to be a smuggling operation. He said the people in the trailer would be reviewed by ICE agents to determine their immigration status. ICE is leading the investigation.

"We’re looking at a human trafficking crime," McManus said.

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.

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