Dozens of people were found dead in and near a tractor-trailer and 16 were taken to hospitals in a presumed migrant smuggling attempt into the United States, said officials in San Antonio, Texas.
The number of fatalities was initially put at 46 but was subsequently raised to 51.
It’s among the deadliest tragedies to have claimed thousands of lives of people attempting to cross the U.S. border from Mexico in recent decades.
Ten migrants died in 2017 after being trapped inside a truck that was parked at a Walmart in San Antonio. In 2003, 19 migrants were found in a sweltering truck southeast of San Antonio.
Police said a city worker at the scene on a remote back road in southwest San Antonio was alerted to the situation by a cry for help shortly before 6 p.m. on Monday.
"A worker, who is works in one of the buildings up here behind me, heard a cry for help. Came out to investigate. Found a trailer with the doors partially opened. Opened them up to take a look. And found a number of deceased individuals inside," Police Chief William McManus said.
Officers arrived to find a body on the ground outside the trailer and a partially opened gate to the trailer, he said.
Temperatures in San Antonio, which is about 250 kilometres from the Mexican border, swelled to a high of over 39 degrees Celsius on Monday with high humidity.
A San Antonio Fire Department official said they found "stacks of bodies" and no signs of water in the truck, which was found next to railway tracks in a remote area on the city's southern outskirts.
Of the 16 taken to hospitals with heat-related illnesses, 12 were adults and four were children, said Fire Chief Charles Hood. The patients were hot to the touch and dehydrated, and no water was found in the trailer, he said.
“They were suffering from heat stroke and exhaustion," Hood said. "It was a refrigerated tractor-trailer, but there was no visible working AC unit on that rig.”
Those in the trailer were part of a presumed migrant smuggling attempt into the United States, and the investigation was being led by U.S. Homeland Security Investigations, McManus said.
Three people were taken into custody, but it was unclear if they were absolutely connected with human trafficking, McManus said, adding that it was now a federal investigation.