A driver has been arrested after eight people were found dead in a tractor trailer loaded with at least 30 others outside a Walmart store in Texas.
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 did not have a working air conditioning system despite blistering temperatures. 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. "It was a mass casualty situation for us."
A person from the truck initially approached a Walmart employee in a car park and asked for water late on Saturday night or early on Sunday morning, said police in San Antonio, where temperatures on Saturday reached 38C (101F).
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, Mr McManus said, but he did not release their 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," Mr 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 US 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 38C (101F) just before 5pm on Saturday and did not dip below 32C (90F) until after 10pm.
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.