San Antonio - Authorities called to a Walmart parking lot in San Antonio overnight found eight people dead and 20 others in dire condition in the back of a sweltering tractor-trailer, in what police are calling a horrific case of immigrant smuggling.The truck's driver was arrested and all 28 survivors were taken to hospitals, where 20 were in extremely critical or serious condition, authorities said. Eight others were being treated for lesser injuries, including heat stroke and dehydration.Temperatures in San Antonio reached 38°C on Saturday and didn't dip below 32°C until after 22:00, according to the National Weather Service. The truck's trailer also didn't have a working air conditioner system, San Antonio Fire Chief Charles Hood said in a news briefing."They were very hot to the touch. So these people were in this trailer without any signs of any type of water," he said. "It was a mass casualty situation for us."A person from the truck initially approached a Walmart employee in the parking lot and asked for water late on Saturday night or early on Sunday morning, police Chief William McManus said. The employee gave the person the water and then called police, who found the dead and desperate inside the truck.McManus said the driver was arrested, but he didn't release the driver's name.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 that there were also what appears to be two school-age children, as well. He called the case "a horrific tragedy".The US Department of Homeland Security is assisting in the investigation.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 re-sentenced in 2011 to nearly 34 years in prison after a federal appeals court overturned the multiple life sentences he had received.