San Antonio: Outside a Walmart in San Antonio on Saturday night, police said, they discovered a sweltering tractor-trailer with dozens of people locked inside - eight of them dead and many more with signs of severe dehydration.

Juveniles appeared to be among the dead - numbered and tagged but still unidentified.

"They discovered an alien smuggling venture gone horribly wrong. Eight immigrants were found dead," US Attorney Richard Durbin wrote in a statement released by federal immigration authorities on Sunday morning. "All were victims of ruthless human smugglers indifferent to the well-being of their fragile cargo."

Police Chief William McManus did not go quite so far when he spoke to reporters gathered before dawn. But he said his homicide detective would work with federal immigration authorities to determine "the origin of this horrific tragedy."

The truck had no working air conditioning or signs of water as it sat in the Walmart parking lot off Interstate Highway 35 in south San Antonio, about two hours from the border with Mexico, authorities said.

Surveillance footage recorded vehicles pulling up to the truck that night, taking people from the trailer and driving away, McManus said.

But at least three dozen remained locked inside, Fire Chief Charles Hood told reporters, their hearts beating rapidly and their temperatures spiking.

At some point, somehow, one of the passengers got out of the trailer and asked a Walmart employee for water.

The employee "came back with the water, called the police, and we found eight dead in the back of that trailer," McManus said.

Some of the survivors ran into the surrounding trees, the police chief said, evading helicopters and foot patrols in the darkness.

But many more remained, badly needing help.

"They were very hot to the touch," Hood said. "Each one of them had over 130 beats per minutes."

Twenty people were hospitalised in critical or very serious condition, Hood said, with many suffering from heat stroke and dehydration.

Eight were in better shape but still needed treatment.

"We flooded downtown San Antonio and our critical hospitals with patients tonight," Hood said.

A hearse pulled into the parking lot later that morning, according to the San Antonio Express-News. The truck and several police cars still sat outside the store.

The news outlet reported that 17 of those hospitalised had life-threatening injuries and that two 15-year-olds were among the injured.

The names, origins and ages of the dead were not known, but McManus said they appeared to include "some juveniles, some adults in their 20s, 30s."

At least two in the truck were school-aged children, Hood said, without specifying whether they were alive.

McManus said the driver was in custody. While investigators did not yet know where the truck came from, the police chief said human smuggling operations were routine in the area - often going undiscovered in the darkness.

"We're very fortunate there weren't 38 people that were all locked inside the vehicle, dead," Hood added.

Washington Post