By Matt Wotus and Michelle Rice, CNN

(CNN) -- The driver of a tractor-trailer found in July packed with dozens of undocumented immigrants, 10 of whom died, pleaded guilty to federal charges in San Antonio on Monday and faces up to life in prison.

James Matthew Bradley Jr., 61, pleaded guilty to one count of conspiracy to transport aliens resulting in death and one count of transporting aliens resulting in death, according to a Department of Justice news release.

By pleading guilty, Bradley admitted that "he conspired to transport and did transport undocumented aliens in the United States for financial gain ... doing so with reckless disregard that they entered this country illegally," which resulted in 10 deaths, the Justice Department said.

Pedro Silva Segura, a 47-year-old undocumented immigrant living in Laredo, Texas, and Bradley's co-defendant, faces four charges, including two counts of transporting undocumented aliens resulting in serious bodily injury and placing lives in jeopardy. Silva is in custody and will be transferred to San Antonio. Others were charged as material witnesses.

On the morning of July 23, authorities found the tractor-trailer parked at a Walmart in San Antonio after a store employee called police for a welfare check. The employee became concerned after a man from the trailer asked for water, the police chief said.

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

Officials found eight bodies inside the tractor-trailer and 31 people suffering from various injuries. Two more died after being hospitalized.

The air conditioner in the trailer was not working, officials said. The high temperature in San Antonio the previous day was 100 degrees Fahrenheit (38 degrees Celsius).

"We quickly called a 'mass casualty incident' and had about 29 units arrive out there and start transporting people," 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."

"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."

Bradley originally said he was not aware of the cargo in his vehicle and only discovered it when he parked at the Walmart and went outside to urinate. A federal grand jury in Texas indicted him in August.

Bradley remains in custody and will be sentenced on January 22, 2018.

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