SAN ANTONIO (CNN) — James Matthew Bradley Jr. says he had no idea dozens of undocumented immigrants were stuffed inside the brutally hot semi truck he was driving.

That’s the story he told federal investigators, who clearly don’t believe him. The 60-year-old Florida man is charged with knowingly transporting undocumented immigrants. The assistant public federal defender who is representing Bradley couldn’t be reached for comment on Monday.

Bradley had been driving the tractor-trailer found parked at a Walmart in San Antonio early Sunday morning. After an employee spotted the vehicle, thought it looked suspicious and called police, authorities found dozens of undocumented immigrants inside.

Eight people in the truck were already dead, and two more died after being hospitalized. Dozens more were severely injured, and many will suffer from “irreversible brain damage,” the city’s fire chief said.

San Antonio’s police chief called the case “a human trafficking crime.”

“Checking the video from the store, we found there were a number of vehicles that came in and picked up a lot of the folks that were in that trailer that survived the trip,” Police Chief William McManus said.

But the truck driver said he didn’t know what — or who — he was hauling.

Driver said he was surprised by ‘Spanish’ people

When police came to investigate the semi, an officer found “multiple people standing and laying at and around the rear of the trailer,” according to a criminal complaint against Bradley.

That’s when they started talking to the driver.

“Bradley advised the officer that the trailer he was hauling had been sold and he was transporting the trailer from Schaller, Iowa, to Brownsville, Texas,” on the border with Mexico, the federal criminal complaint says. Bradley said he was delivering the truck to its new owner in Brownsville at his boss’s request, but that he was not given a time frame nor a delivery address, the complaint said.

Bradley later told authorities he was traveling to San Antonio from Laredo, Texas after getting the tractor-trailer washed and detailed, the complaint said. After the wash at the Laredo truck stop, Bradley said he drove to the “old truck stop,” which was about 10 miles away, to have the tractor-trailer detailed and polished, the complaint said.

Pat Sullivan, a spokesman for Blue Beacon Truck Wash told CNN that the tractor-trailer was washed at their facility on Beltway Parkway in Laredo at 11:58 a.m. on Saturday. Sullivan said drivers typically pull up, pay inside of the facility and then workers wash the truck, which takes about 20 minutes.

Sullivan said the employees who washed the truck did not report hearing any noises from inside the trailer or notice anything strange about the truck. The company is investigating and cooperating with authorities, Sullivan said.

“Bradley stated he was unaware of the contents and/or cargo,” the complaint said, until he parked the tractor-trailer at the Walmart and went outside to urinate. That’s when he heard movements in the trailer, the driver told investigators.

“Bradley said he went to open the doors and was surprised when he was run over by ‘Spanish’ people and knocked to the ground,” the complaint states.

“Bradley said he then noticed bodies just lying on the floor like meat. Bradley said he knew at least one of them was dead. Bradley said he knew the trailer refrigeration system didn’t work and that the four vent holes were probably clogged up.”

But the driver didn’t call 911, authorities said. Instead, he went back to the tractor and called his wife.

Bradley was in handcuffs as he limped into a federal court on Monday, surrounded by several US marshals. The driver spoke briefly with his assistant public federal defender before the arraignment. Bradley’s detention and request for bail will be addressed at a preliminary hearing on Thursday.

A horrific journey

Homeland Security Investigations agents interviewed several of the undocumented immigrants who had been hospitalized.

One said he left the central Mexican city of Aguascalientes and waited with about 28 other people in Nuevo Laredo to be smuggled across the river into the United States, bound for San Antonio.

He told investigators he paid about 12,500 pesos ($707) for protection and a raft ride across a deep portion of the Rio Grande. After crossing at night, “they walked until the next day,” the court document states.

At 9 a.m the next day, his group was picked up in a silver Chevrolet Silverado truck and taken to the trailer, which already had about 70 people inside, the criminal complaint says.

“He was told to get inside and he would be transported later that evening. The smugglers closed the door and the interior of the trailer was pitch black and it was already hot inside.”

With no food or water, the migrants tried to make noise and get the driver’s attention, “but nobody ever came.”

Twelve hours later, they were moved to another trailer, the man said. Some people had trouble breathing and passed out.

When the trailer eventually braked hard and stopped, some people fell over because they were so weak, the man said. After the door opened, six black SUVs were waiting to pick up the people, but those SUVs quickly filled up and took off.

The man said once he arrived in San Antonio, he was supposed to pay his smugglers $5,500. But instead he was taken to a hospital, severely injured from the journey.

“These people were helpless in the hands of their transporters,” said Richard L. Durbin Jr., US attorney for the Western District of Texas. “All were victims of ruthless human smugglers indifferent to the well-being of their fragile cargo.”

Authorities have not released the nationalities of the people crammed inside the tractor-trailer. But the Mexican government issued a statement offering its condolences to the families of the victims.

“Mexico’s consulate in San Antonio is working closely with local and federal authorities, hospitals and coroners to learn the victims’ nationalities and to provide the necessary consular protection and assistance, including the repatriation of the remains of Mexican victims, if any,” the statement reads.

‘Irreversible brain damage’

The tip-off to authorities started when a man from the trailer asked a Walmart employee for water, the police chief said. The employee was concerned and called police for a welfare check.

The fire department also responded and soon declared a “mass casualty incident,” San Antonio Fire Chief Charles Hood said.

“With heat strokes or heat injuries, a lot of them are going to have some irreversible brain damage,” he said.

A heatstroke can cause swelling of the brain and other vital organs, possibly causing permanent damage, if a person’s body temperature isn’t quickly lowered, according to the Mayo Clinic.

Officials said the air conditioner in the trailer was not working. And the high temperature in San Antonio on Saturday was 100 degrees Fahrenheit (38 degrees Celsius).

“Unfortunately, some of them were severely overheated, and that was a refrigerated truck with no refrigeration,” Hood said. “So we were very fortunate that they were found.”

Two of the people hospitalized are 15 years old, the fire department said.

Echoes of 2003 case

In a state where human smuggling happens with alarming frequency, Sunday’s discovery stirred memories of a 2003 case in Victoria, Texas, that left 19 people dead. At the time, a border and transportation security official called it “the greatest loss of life in recent history in what appears to be an alien smuggling case.”

The victims, ranging from a 7-year-old boy to a 91-year-old man, died of asphyxiation, dehydration or heat. They had been crowded in the back of a truck with about 100 other people, with temperatures soaring past 100 degrees Fahrenheit, investigators said.

The driver in that case, Tyrone Mapletoft Williams of Schenectady, New York, was initially sentenced to life in prison, but in 2011 was resentenced to almost 34 years in prison.

Thomas Homan, the current acting director for Immigration and Customs Enforcement, was a lead investigator in the Victoria case.

Last month, he disputed the notion that his policies were heartless and said the humane thing to do is to deter people from paying the human smugglers and cartels that are the only way to illegally cross the Southwest border.

“Why am I so strong in what I’m trying to do? Because people haven’t seen what Tom Homan’s seen,” Homan said.

“They haven’t seen the dead immigrants on a trail that were left stranded. … People weren’t standing with me in Victoria, Texas, in the back of a tractor-trailer with 19 dead aliens including a 5-year-old child laying dead under his father that suffocated.”

‘Human trafficking is an epidemic’

Texas Gov. Greg Abbott said the deaths were “a heartbreaking tragedy.”

US Rep. Lloyd Doggett, a Democrat who represents parts of San Antonio, called for reform.

“Horrific scene overnight in San Antonio where a human smuggling attempt turned deadly,” Doggett tweeted.

“Prosecute smugglers, pray for survivors and the victims’ families, stop the hysteria, reform our broken immigration system.”