SAN ANTONIO

Officials in Mexico also released a surveillance photo showing the driver smiling at the checkpoint during the more than two-hour trip to San Antonio.

Authorities were looking into whether the truck had mechanical problems when it was left next to a railroad track. The driver was apprehended after trying to disguise himself as one of the migrants, Garduño said.

Some of the more than a dozen people transported to hospitals were found suffering from injuries such as brain damage and internal bleeding, according to Rubén Minutti, the Mexico consul general in San Antonio.

Migrants typically pay $8,000 to $10,000 to be taken across the border, loaded into a tractor-trailer and driven to San Antonio, where they transfer to smaller vehicles for their final destinations across the United States, said Craig Larrabee, acting special agent in charge of Homeland Security Investigations in San Antonio.

The death count from Monday's tragedy in San Antonio was the highest ever from a smuggling attempt in the U.S., he said. Four years ago, ten died in 2017 after being trapped inside a truck parked at a San Antonio Walmart. In 2003, the bodies of 19 migrants were found in a sweltering truck southeast of the city.

Temperatures in San Antonio on Monday approached 100 degrees Fahrenheit (38 degrees Celsius), and those taken to the hospital were hot to the touch and dehydrated, authorities said.

It wouldn't have taken long for the temperature inside the truck to become deadly, said Jennifer Vanos, an assistant professor at Arizona State University who has

researched child deaths in hot vehicles.

The tractor-trailer likely would have been hot even before anyone got inside and because of the high humidity, lack of air flow and so many people inside, their bodies could not cool through evaporation and they would have dehydrated quickly, she said.

With little information about the victims, desperate families of migrants from Mexico and Central America have frantically sought word of their loved ones.

Felicitos Garcia, who owns a small grocery store in the remote community of San Miguel Huautla in Mexico's southern state of Oaxaca, said the mother of Jose Luis Vasquez Guzman, who was hospitalized in Texas, had gone to the state capital to learn more about her son's condition and the whereabouts of his cousin, who is believed to be missing.

“Life is tough here,” Garcia said. “People survive by growing their own crops like corn, beans and wheat. Sometimes the land gives and sometimes it doesn't when the rains arrive late. There is nothing in place for people to have other resources. People live one day to the next.”

The process of tracking down the victims is painstaking because among the pitfalls are fake or stolen documents.

Mexico's foreign affairs secretary identified two people Tuesday who were hospitalized in San Antonio. But it turned out that one of the identification cards he shared on Twitter had been stolen last year in the southern state of Chiapas.

Haneydi Antonio Guzman, 23, was safe in a mountain community more than 1,300 miles (2,092 kilometers) away from San Antonio

when she began receiving messages from family and friends anxious over her fate. There is no phone signal there, but she has internet access.

Journalists started showing up at her parents' home in Escuintla — the address on her stolen ID that was found in the truck — expecting to find her worried relatives.

“That's me on the ID, but I am not the person that was in the trailer and they say is hospitalized,” Antonio Guzman said.

“My relatives were contacting me worried, asking where I was,” she said. “I told them I was fine, that I was in my house and I clarified it on my” Facebook page.

In some regions of Mexico, attempting to cross into the United States is such a tradition that most youths in heavily migrant towns at least consider it.

“All of the young people start to think about going (to the U.S.) as soon as they turn 18,” said migrant activist Carmelo Castañeda, who works with the nonprofit Casa del Migrante. “If there aren't more visas, our people are going to keep dying.”