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SAN ANTONIO — The truck driver accused in what authorities are calling a "horrific" human smuggling operation has been charged and could face life in prison or the death penalty if convicted.

The suspect, James Matthew Bradley Jr., 60, of Clearwater, Fla., appeared briefly in court Monday morning, hours after officials announced that a 10th victim in the case had died.

Bradley was taken into custody after authorities made a gruesome discovery over the weekend: dozens of suspected migrants crammed into the back of an overcrowded, sweltering 18-wheeler in a San Antonio, Texas, parking lot.

Initially, eight were dead inside the overheated trailer when authorities found them, and more than a dozen others were suffering from life-threatening injuries. The two additional deaths happened at the hospital, authorities said.

The disturbing scene unfolded in the late hours of Saturday night and early hours of Sunday morning when an unidentified tipster, who had been in the truck, approached a Walmart security guard and asked for water, San Antonio Police Chief William McManus told reporters, describing it as a "horrific tragedy."

Bradley is charged with one count of transporting undocumented immmigrants "for the purpose of commercial advantage or private financial gain to wit," a federal criminal complaint filed Monday morning says.

James Mathew Bradley Jr. arrives at the federal courthouse for a hearing on July 24 in San Antonio, Texas. Eric Gay / AP

According to the complaint, Bradley told San Antonio police officers he was driving from Schaller, Iowa, to Brownsville, Texas, to deliver his trailer to a new owner after his boss sold it and asked him to deliver it.

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Bradley said he had no idea what was inside the truck until he took a bathroom break and heard movement inside.

He "stated when he arrived at the Walmart he exited the vehicle to urinate and he heard banging and shaking in the trailer," according to the complaint. "Bradley said he went to open the doors and was surprised when he was run over by 'Spanish' people and knocked to the ground. Bradley said he then noticed bodies just lying on the floor like meat."

Aware that the refrigeration system wasn't working, he then went back to the tractor to call his wife, but did not call 911, the complaint says.

About three dozen people were in or around the truck when first responders arrived. Fire Chief Charles Hood told reporters Sunday that those inside were "very hot to the touch."

"Our paramedics and firefighters found that each one of them had heart rates over about 130 beats per minute," he said. "You're looking at a lot of heat stroke, a lot of dehydration."

Brian Pyle, the owner of the Iowa-based company advertised on the side of the truck, Pyle Transportation, told NBC News he had sold the truck in May.

“I had nothing to do with anything that guy did,” Pyle said. “I had no connection, had no idea what he was doing. I feel bad for what happened down there. I’m sorry for everything that happened. I found out about it on the news like everyone else.”

It was not immediately clear where the victims were from, but Mexico's consul general, Rayna Torres, confirmed Sunday that Mexican nationals were among them. The Guatemalan Foreign Ministry said Monday that two of the hospitalized migrants — ages 17 and 23 — were from Guatemala. They are in stable condition. At least one of the deceased was also from that country, the Foreign Ministry said.

The Department of Homeland Security was working with local police in the case. Homeland security agents interviewed one of the undocumented immigrants at the hospital, who told them he came from Aguascalientes, Mexico, and was supposed to pay smugglers $5,500 once he reached San Antonio, according to the complaint.

The migrant detailed a long journey involving crossing the Rio Grande River into the U.S. by raft, walking for a day, getting picked up by a Chevrolet Silverado, and then being thrown into the pitch-black trailer where he was discovered with no water or food.

"During the first hour of transportation, everyone seemed to be ok. Later, people started having trouble breathing and some started to pass out. People began hitting the trailer walls and making noise to get the driver's attention. The driver never stopped," the complaint says.

Adan Lalravega, a 27-year-old laborer from Aguascalientes, told the Associated Press that he lost consciousness amid the sounds of whimpering children and pleas for water. When Lalravega awoke he was in a San Antonio hospital bed.

Officials believe up to 200 migrants may have been transported at different times as part of the operation.

"To maximize their criminal profits, these human smugglers crammed more than 100 people into a tractor trailer in the stifling Texas summer heat resulting in ten dead and 29 others hospitalized," Acting Immigration and Customs Enforcement Director Thomas Homan said in a press release from the U.S. attorney's office for Western Texas.

Bradley's next court appearance is set for Thursday. He has requested a court-appointed attorney.

Matthew Vann reported from San Antonio, Elizabeth Chuck reported from New York, Tim Stelloh reported from San Francisco.