Police made the disturbing discovery on Saturday after a man managed to get out of the truck and ask a Walmart employee for water, San Antonio Police Chief William McManus said. The employee gave the man water and then called cops.
"We're looking at a human trafficking crime," McManus said.
Two 15-year-olds were the youngest victims. Most appeared to be in their 20s and 30s, according to McManus.
"They were very hot to the touch. So these people were in this trailer without any signs of any type of water," Fire Chief Charles Hood said. "It was a mass casualty situation for us."
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The truck's driver, James Matthew Bradley Jr, 60, who earlier appeared in federal court, is charged with one count of transporting immigrants who are in the country illegally and faces up to life in prison or the death penalty under a law that allows enhanced penalties when immigrants die during smuggling operations.
The air conditioner in the trailer apparently was not working. Seventeen of those rescued were listed in critical condition while 13 others in serious condition.
Hood said many of the survivors will have irreversible brain damage.
Once the immigrants have received medical treatment, police said they will be handed over to the US Immigration and Customs Enforcement agency.
Officials revised an earlier count of the number of people in the trailer after a 39th person was found in a nearby area.
Temperatures reached 101 degrees on Saturday. The stifling heat did not drop below 90 degrees until well into the evening. At 80 degrees outside, the temperature inside closed vehicles can reach 125 degrees in an hour, experts say.
"Based on initial interviews with survivors, there may have been more than 100 people in the trailer, many of whom are thought to have fled or been picked up," said Thomas Homan, acting director of US Immigration and Customs Enforcement.
Surveillance video showed vehicles had pulled up, taken people from the tractor-trailer and driven away, McManus said.
Governor Greg Abbott said in a written statement that the state would "continue to provide protection for the victims who have been robbed of their most basic rights and bring down the full weight of the law for the perpetrators of this despicable crime."