Two people were found dead and at least 10 others were taken to hospital after “suffocating” in a Texas-bound freight train.
Police in Uvalde, Texas, were alerted to the situation on Friday afternoon after receiving a call from an unknown person seeking help. They were able to stop the service while it was travelling near the US-Mexico border to San Antonio.
Officers subsequently found 12 migrants in a shipping container – including the two who died. A further three people were found in a separate carriage.
At least four were then airlifted to a hospital in San Antonio.
“We’re still trying to determine if [the call] was from someone inside the car,” police chief Daniel Rodriguez told the San Antonio-Express News.
“We’re assuming it was from inside one of the cars.”
Mr Rodriguez suspected the victims were dehydrated due to the heat of the train cars during warm weather, KSAT television reported.
It’s heartbreaking,” KSAT quoted Rodriguez as saying.
Union Pacific railroad said it was “deeply saddened by [the] incident and the tragedies occurring at the border”, and are set to launch an investigation.
“We take the safety of all individuals seriously and work tirelessly with law enforcement partners to detect illegal items and people riding inside or on our rail cars,” the company said.
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The train was stopped near the town of Knippa, around 72 miles (115km) from the southern border. With highs of 32.2C reported in the local area, the temperature within the container is thought to have been much hotter.
In a statement, Homeland Security Secretary Alejandro Mayorkas suggested the deaths and injuries were linked to human trafficking and pledged to hold those responsible accountable.
“We are heartbroken to learn of yet another tragic incident of migrants taking the dangerous journey,” Mr Mayorkas said on Twitter. “Smugglers are callous and only care about making a profit.”
Last summer, more than 50 people died after dozens were found in the back of an abandoned tractor-trailer on the outskirts of San Antonio.
