 -- An undocumented immigrant who survived an alleged smuggling attempt on a stifling hot tractor-trailer where 10 people died told investigators that the driver ignored their banging from inside a shipping container as they took turns breathing through a hole in the wall, according to a criminal complaint.

Some people had trouble breathing and passed out, the man said, according to a criminal complaint filed today against the driver that does not name the immigrant.

The immigrant told investigators there were about 70 other people when he and his group of 28 got inside the tractor-trailer, where officials said temperatures climbed to over 100 degrees, according to the criminal complaint. When authorities responded to the tractor-trailer parked outside a San Antonio, Texas, Walmart this weekend, a total of 39 people, including children, were found inside. Ten people, all adult men, died, and 29 people were being treated at hospitals, officials said. Many experienced heat stroke and dehydration, officials said.

The immigrant told investigators he was from Mexico and was being smuggled into the United States with a final destination of San Antonio, according to the criminal complaint.

"He stated once he arrived in San Antonio, TX he was to pay the smugglers," the document said, adding that he told authorities the pitch-black container had no water or food.

When the rear doors were finally opened, the undocumented immigrant told investigators, people swarmed out and six black SUVs were waiting to pick them up. The vehicles filled up in a matter of minutes before leaving, the criminal complaint said. It’s unclear why he stayed behind.

The truck was discovered early-Sunday morning by San Antonio firefighters and police after a Walmart employee called late-Saturday for a welfare check when someone asked the employee for water, officials said.

The responding authorities "discovered an alien-smuggling venture gone horribly wrong," U.S. Attorney Richard Durbin Jr. of the Western District of Texas said in a statement.

"All were victims of ruthless human smugglers indifferent to the well-being of their fragile cargo,” he said. “The South Texas heat is punishing this time of year. These people were helpless in the hands of their transporters. Imagine their suffering, trapped in a stifling trailer in 100-plus-degree heat."

The criminal complaint alleges that when a San Antonio police officer responded to the scene, the driver, James Mathew Bradley Jr., 60, exited the tractor-trailer and told the officer that the trailer he was hauling had been sold and he was taking it from Iowa to Brownsville, Texas.

Bradley told the officer he was unaware of the contents, the criminal complaint said.

Bradley told the officer "after he parked his tractor trailer he exited the vehicle to urinate when he heard movement in the trailer. Bradley said he then went to the rear of the trailer and opened the door. Bradley stated he tried to administer aid to the occupants," according to the document.

According to the criminal complaint, Bradley said his boss told him he had sold the trailer to a person in Brownsville and asked him to deliver it. Bradley said he was not given a time frame to deliver the trailer nor was he given the delivery address, according to the criminal complaint.

"Bradley stated when he arrived at the Wal-Mart he exited the vehicle to urinate and he heard banging and shaking in the trailer," the criminal complaint said. "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."

Bradley said he went back to the tractor-trailer and called his wife but she didn't answer, the criminal complaint said. He said he did not call 911. He said about 30 to 40 people ran out of the trailer when he opened the doors.

The criminal complaint alleges Bradley "knowing or in reckless disregard of the fact that aliens had come to, entered or remained in the United States in violation of law, transport and move and attempt to transport and move such aliens for the purpose of commercial advantage of private financial gain ... and approximately thirty six others within the United States by means of transportation and otherwise, in furtherance of such violation of law resulting in the death of approximately 10 of the aliens so transported."

Bradley, of Clearwater, Florida, made his first appearance in federal court today; he did not make any statements beyond saying "yes" to the judge as she explained the charges and that he would also need a death penalty attorney. Bradley has a court appointed attorney, who declined to comment.

His next hearing is scheduled for July 27. According to the criminal complaint, the maximum sentence if convicted is the death penalty or life in prison.

Acting Director Thomas Homan of U.S. Immigration and Customs Enforcement said in a statement that the "horrific crime" is a "stark reminder of why human smuggling networks must be pursued, caught and punished."

ABC News' Kenneth Moton contributed to this report.