SAN ANTONIO (AP) — Eight people were found dead in a tractor-trailer outside a Walmart store in what police called a horrific human trafficking case, and the driver was arrested.

Several other people, possibly dozens, were found in the truck, which didn't have a working air conditioning system, and they were taken to hospitals, firefighters said. Authorities said the driver had been held, but they didn't release the driver's identity.

A person from the truck approached a Walmart employee in a parking lot and asked for water late Saturday night or early Sunday morning, San Antonio police said. 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, police Chief William McManus said.

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.

He called the case "a horrific tragedy."

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