The driver and any other suspects could face both state and federal charges, authorities said.
This and other tragic instances of human smuggling, including a 2003 case in Victoria, Texas, in which 19 immigrants died, highlight the dangers that extreme heat poses to would-be immigrants. "These people were helpless in the hands of their transporters", Durbin said.
"Imagine their suffering, trapped in a stifling trailer in 100-plus degree heat", he said.
The US Department of Homeland Security was involved in the investigation into what happened.
Hours later, after daybreak, a helicopter hovered over the area, and investigators were still gathering evidence from the tractor-trailer, which had an Iowa license plate and was registered to Pyle Transportation Inc. of Schaller, Iowa. He later added that emergency workers used triage techniques to help people in what he called a "mass-casualty" event. A Walmart security guard then found the bodies and alerted authorities.
Authorities said the driver had been held, but they didn't release the driver's identity.
Police Chief William McManus says some of those in the truck ran into the woods, leading to a search. He said the victims include "some juveniles", along with adults in their 20s and 30s.
San Antonio police say several other people from the tractor-trailer have been taken to hospitals. Dozens more people in the trailer were dealing with heat-related illnesses. "As Chief McManus mentioned, we do have at least two school-aged children".
"We are very fortunate that there weren't 38 people who were locked inside of this vehicle dead", he said.
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He called it "very sad". Most of the migrants were from Central America, and 48 of them were minors.
The truck was parked in a Walmart parking lot. The employee gave them water and called 911.
Owner Brian Pyle told The Washington Post that the Louisville, Kentucky-based driver owned the truck and was on his first trip.
As it was, Hood said, 30 were hospitalized - 17 in critical condition.
"We are still learning about the tragic events that unfolded on our doorstep overnight, which shines a bright light on the plight of immigrants looking for a better life and victims of human trafficking".
- August 27, 2015: Austrian police discovered an abandoned truck containing the bodies of 71 migrants, including eight children, from Iraq, Syria and Afghanistan. The store's surveillance video is part of the ongoing investigation, McManus said.
The CDC says about 618 people die in the US each year from heat-related illnesses.
"For those people who survived, they took a beating", Hood told CNN.
It was just the latest smuggling-by-truck attempt to end in tragedy. This happens quite frequently.