Six people were found dead on Sunday in a cargo train boxcar in southern Texas, US media reported.
The bodies were discovered at a rail yard near the US-Mexico border by an employee for Union Pacific, a freight-hauling railroad company, the Laredo Police Department said, according to the New York Times.
Police were investigating how the individuals died, the Times said.
"It's a very unfortunate event ... It was too many lives that were lost," Jose Espinoza, a public information officer with the police, said, according to CNN.
Temperatures near where the train was stopped reached 90F (32C) on Sunday.
Two migrants died after being found trapped in a train in Texas in 2023.
A year earlier, 53 migrants died after being abandoned in a scorching-hot tractor-trailer on an isolated Texas road.