The employee "came back with the water, called the police, and we found eight dead in the back of that trailer", McManus said.
In Mexico, smugglers have often crammed migrants bound for the United States into tractor-trailers, sometimes in hidden compartments. A Walmart security guard then found the bodies and alerted authorities. "We're looking at a human trafficking crime here", said McManus. "Homicide will work to determine the origin of this horrific tragedy".
He said there appeared to be a mix of juveniles and people in their 20s and 30s inside the trailer.
San Antonio police say several other people from the tractor-trailer have been taken to hospitals.More news: India's Ambani To Launch Free Smartphone As He Shakes Up Telecoms
Prosecutors said Williams originally earned a death sentence because he intentionally caused the immigrants' deaths by not freeing them when he knew their lives were in danger, CBS News reported.
It was not immediately clear how many people made it off the truck alive, McManus said. At least two young children were also found.
He called it "very sad".
Authorities have not said whether the rig was locked when cops arrived, or whether it was used to smuggle the almost 40 people across the border into the US. The driver has been taken into custody.
A hearse pulled into the parking lot early Sunday morning, according to the San Antonio Express-News. The person also reportedly told the employee that there were people inside a trailer who needed help.More news: Trump to Help Commission Next-generation Warship in Virginia
It remains unclear how long the truck was parked in the lot, but authorities say they think internal temps reached an unbearable 130 degrees.
As it was, Hood said, 30 were hospitalized - 17 in critical condition. "So these people were in this trailer without any signs of any water", Fire Chief, Charlie Hood, described in the video to reporters on social media. "Fortunately, we came across this one".
Police were searching areas around the Walmart on Sunday to look for anyone who ran from the truck. A search overnight did not locate any additional victims, but police said they would conduct a second aerial search once daylight broke.
According to McManus, the victims "will be investigated by ICE" after they are treated.
The driver of the vehicle was arrested and could face federal and state charges, officials said.More news: Ann Coulter Exchanges Major Shade with Airline Over Seat Relocation Drama
The CDC says about 618 people die in the US each year from heat-related illnesses. The remaining 8 sustained lesser injuries, such as heat stroke. This happens quite frequently.