McManus said police made the disturbing discovery after a Walmart employee was approached by someone from the truck who asked him for water.
Authorities discovered the grisly scene after they inspected Bradley's truck in a Walmart parking lot. Twenty people were taken to area hospitals in critical condition, and another eight had "less critical" injuries.
"[This] is not an isolated incident", Police Chief William McManus said. All survivors were taken to several hospitals.More news: Newborn killer whale calf dies, SeaWorld San Antonio announces
The U.S. Attorney's office is holding 60-year-old James Bradley, Jr. of Clearwater, Florida in connection with the case, which they're calling a "horrific" human trafficking crime.
It remains unknown where the truck came from and how long it had been parked at the location. At least two young children were also found.
The National Weather Service says the temperature in San Antonio hit 101 degrees just before 5 p.m. Saturday and didn't dip below 90 degrees until after 10 p.m. He says numerous truck's occupants appeared to be in their 20s and 30s but that there were also what appeared to be two school-age children.
These kinds of horrific tragedies are occurring with shocking frequency in San Antonio, which has become a center of human smuggling and trafficking.More news: Evacuation order due to Detwiler Fire lifted for town of Mariposa
Those still alive are expected to have incurred brain damage from the severe heat.
The case will be passed on to US Immigration and Customs Enforcement, the police chief said, once victims have been examined. "And fortunately there are people that survived", McManus said.
Investigators checked store surveillance video, which showed vehicles had arrived and picked up other people from the tractor-trailer, he said.More news: 8 people found dead in tractor-trailer in San Antonio