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."
More From This Section
James Mathew Bradley, 60, a Florida truck driver, was taken into custody andfacesstate and federal charges in connection with the case.
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.
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.
Some of the victims were from Mexico and at least two were from Guatemala, according to officials with both nations.
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."