SAN ANTONIO — Eight people were found dead in a tractor-trailer loaded with at least 30 others outside a Walmart store in Texas' stifling summer heat in what police are calling a horrific human trafficking case. The driver was arrested.

Twenty other people in extremely critical or serious condition and eight more with lesser injuries including heat stroke and dehydration were found inside the truck, which didn't have a working air conditioning system despite blistering temperatures that topped 100 degrees, authorities said. All were taken to several hospitals.

"They were very hot to the touch. So these people were in this trailer without any signs of any type of water," said San Antonio Fire Chief Charles Hood, briefing journalists at the scene in video authorities posted on social media. "It was a mass casualty situation for us."

A person from the truck initially approached a Walmart employee in a parking lot and asked for water late Saturday night or early Sunday morning, said police in San Antonio, where temperatures on Saturday reached 101 degrees. The employee gave the person the water and then called police, and when officers arrived they found the eight people dead in the back of the trailer and 30 other survivors inside, police Chief William McManus said.

The driver had been held, McManus said, but he didn't release the driver's identity.

Investigators checked store surveillance video, which showed vehicles had arrived and picked up other people from the tractor-trailer, police said.

"We're looking at a human trafficking crime this evening," McManus said, adding many of those inside the truck appeared to be adults in their 20s and 30s but also apparently two school-age children. He called the case "a horrific tragedy."

The U.S. Department of Homeland Security was involved in the investigation into what happened, he said.

The National Weather Service's local office said the temperature in San Antonio hit 101 degrees (38 Celsisus) just before 5 p.m. Saturday and didn't dip below 90 degrees until after 10 p.m. Saturday.

Other cases of human trafficking in the United States have led to more deaths. In May 2003, 19 immigrants being transported from South Texas to Houston died inside a sweltering tractor-trailer.

Prosecutors said the driver in the 2003 case heard the immigrants begging and screaming for their lives as they were succumbing to the stifling heat inside his truck but refused to free them. The driver was resentenced in 2011 to nearly 34 years in prison after a federal appeals court overturned the multiple life sentences he had received.