Eight people were found dead on Sunday inside a sweltering tractor trailer parked at a Walmart store in San Antonio, Texas, and authorities said they were the victims of "ruthless" human traffickers.
Another 30 people, many in critical condition and suffering from heat stroke and exhaustion, were with the bodies in the trailer, which lacked air conditioning or a water supply, San Antonio Fire Chief Charles Hood said.
The truck's driver was arrested and will be charged, said Richard Durbin, U.S. Attorney for the Western District of Texas, and prosecutors will work to identify others responsible.
The bodies were discovered after officials were led to the trailer by a man who had approached a Walmart employee and asked for water.
"All were victims of ruthless human smugglers indifferent to the well-being of their fragile cargo," Durbin said.
"These people were helpless in the hands of their transporters. Imagine their suffering, trapped in a stifling trailer in 100-plus (37.8 Celsius) degree heat," he said.
San Antonio is about 240 km from the border with Mexico. Temperatures in the area held above 37.8 Celsius until 6 pm local time yesterday and were expected to soar into the high 30s again today, with humidity making the heat feel close to 40 degrees, forecasters said.
"If you get crammed inside a truck, it's life threatening," said National Weather Service meteorologist Bob Oravec.
Raids on suspected illegal immigrants have ramped up across the United States in recent months, after President Donald Trump's vow to crack down on those entering the country without authorization or overstaying their visas.
San Antonio Police Chief William McManus described the deaths as a "horrible tragedy" and said other suspects had fled the scene as police officers arrived.
"Checking the video, there were a number of vehicles that came and picked up other people who were in that trailer," McManus said.
Twenty people were airlifted to seven hospitals and their conditions were "critical to very critical," Hood said. Eight others are hospitalised in less serious condition, he said.
At San Antonio's University Hospital, six adults were admitted early Sunday with heat-related injuries, all of them in critical condition, spokesman Donald Finley said.
McManus said the people in the truck ranged from school-age juveniles to adults in their 20s and 30s.
He said the Department of Homeland Security had joined the investigation, and that the origin of the truck is unclear.
Experts have warned in recent months that tougher immigration policies could make it more difficult to stop human trafficking. Measures to harden international borders encourage would-be migrants to turn to smugglers and fear of deportation deters whistle-blowing, they said.
While there are no official law enforcement statistics, nearly 32,000 cases of human trafficking in the United States have been reported to the National Human Trafficking Hotline in the last decade.