Two migrants have been found dead and at least 10 others were taken to hospital after police in Texas received a call that they were “suffocating” in a freight train travelling near the US-Mexico border.
The Uvalde Police Department said the Border Patrol was informed of the phone call and able to stop the train.
About 15 migrants were found inside, according to a statement from the department.
Union Pacific railroad said in a statement that the people were found in two cars on the train travelling east from Eagle Pass bound for San Antonio: 12 in a shipping container and three in a “hopper car”.
The two people who died were in the shipping container, the statement said.
At least four people were airlifted to San Antonio, authorities said, while others were taken to local hospitals.
Uvalde police chief Daniel Rodriguez told the San Antonio-Express News that dispatchers received a 911 call about 3.50pm from an unknown person seeking help. The train was stopped near the town of Knippa, which is less than 100 miles from the southern border.
“We’re still trying to determine if it was from someone inside the car,” Mr Rodriguez said. “We’re assuming it was from inside one of the cars.”
Uvalde police said that Union Pacific railroad would lead the investigation.
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.”
Highs in the area neared 32.2C, and shipping containers often get much hotter than the surrounding temperature.
Last summer, more than 50 migrants died after dozens of people were found in the back of a sweltering tractor-trailer that had been abandoned on the outskirts of San Antonio.
The tragedy was the nation’s deadliest smuggling episode on America’s southern border, which led to officials vowing to step up policing efforts.
Migrants routinely travel through Uvalde, leading to high-speed vehicle pursuits that put schools in the area on lockdown.
After the Robb Elementary School massacre in Uvalde last May, when a gunman killed 19 children and two teachers, Texas politicians concluded in a report that the frequency of the lockdowns may have led to a “diminished sense of vigilance” about security.
Union Pacific said it was “deeply saddened by this incident and the tragedies occurring at the border”. It added: “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.”