Nine people, believed to be illegal immigrants being smuggled into the United States, have been found dead inside a lorry in a car park in San Antonio, Texas.
Another 30 people, many suffering from heat stroke and exhaustion, were found with the bodies in the trailer, which lacked air conditioning or a water supply, San Antonio Fire Chief Charles Hood said.
The lorry's driver was arrested and will be charged, said the US Attorney for the Western District of Texas said, while prosecutors will work to identify others responsible.
"These people were helpless in the hands of their transporters," he said in a statement, adding the heat in southern Texas is "punishing" at this time of year.
"Imagine their suffering, trapped in a stifling trailer in 100-plus degree heat," he said.
They were victims of "ruthless human smugglers indifferent to the well-being of their fragile cargo," he added.
San Antonio Police Chief William McManus described it 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," Mr McManus said.
20 people were airlifted to seven hospitals and their conditions were "critical to very critical," Mr Hood said. Eight others are hospitalised in less serious condition, he said.
The people range from school-age juveniles to adults in their twenties and thirties, he said.
Mr McManus said officials were led to the scene by a man who approached a store employee asking for water.
Mr McManus said the Department of Homeland Security had joined the investigation. The origin of the truck is unclear, he added.