Two migrants have been found dead and 10 were hospitalised after police received a panicked call from a group "suffocating" inside a shipping container.
Cops in South Texas, US, were alerted of a distressing phone call from an unknown number as people desperately tried to get help while inside a freight train.
Officers quickly contacted Border Patrol who were able to stop the train which was travelling near the US-Mexico border to San Antonio.
12 people were found in a shipping container and a further three were found in a freight train, according to a statement from the department.
The two people who died were in the shipping container, authorities confirmed.
At least four people were air-lifted to San Antonio, authorities said, while others were taken to local hospitals.
The condition of all of those hospitalised was not immediately known.
University Health in San Antonio tweeted that it had received two male patients, one in critical condition and one in serious condition.
Uvalde Police Chief Daniel Rodriguez told the San Antonio-Express News that dispatchers received an emergency call around 3:50 p.m. 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.
Homeland Security Secretary Alejandro Mayorkas believes the deaths and injuries were linked to human trafficking and pledged to hold those responsible accountable.
The cause of death was not immediately clear, however, temperatures neared 90C and shipping containers often get much hotter in warmer weather.
“We are heartbroken to learn of yet another tragic incident of migrants taking the dangerous journey,” Mayorkas said on Twitter.
“Smugglers are callous and only care about making a profit.”
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.”
The tragic deaths come after 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 often put schools in the area on lockdown.
