At least 38 people were in the trailer including no fewer than two school-age children, Fire Chief Charles Hood said.

After the discovery, eight were found dead while 30 others were hospitalized, in what police said appeared to be a "horrific" human trafficking crime.

Twenty of those taken to area hospitals after the discovery in San Antonio -- about a two hour drive from the Mexican border -- were in serious to "extremely critical" condition, suffering from heat stroke and dehydration, authorities said.

City police chief William McManus said the truck driver had been arrested.

It was not immediately clear how many might have escaped and were unaccounted for.

"We got a call from a Walmart employee about a welfare check in a tractor-trailer that was parked on the lot here," McManus told a news conference.

"He was approached by someone from that truck, who was asking for water."

The employee returned with the water and then called the police who "found eight people dead in the back of that trailer," the police chief said, calling it a "horrific tragedy."

"We're looking at a human trafficking crime this evening," McManus said.