Two suspected undocumented immigrants suffocated to death aboard a freight train and 10 others in need of medical care were taken by helicopter or ambulance to hospital on Friday in south Texas, police said.

Officials in Uvlade, Texas, received an anonymous emergency 911 call advising them that numerous immigrants were suffocating inside a train, Uvalde police said in a statement. At least 15 immigrants needed immediate medical attention, police said.

U.S. Border Patrol officials were notified and able to stop the train just east of Knippa, Texas, in Uvalde County, police said. Officials closed U.S. Highway 90 temporarily to land the helicopters.

Federal investigators were looking into the possibility of human smuggling, investigators with the U.S. Department of Homeland Security said.

The tragedy occurred near the site of a more grave incident last year, when 53 migrants died in the back of a tractor trailer amid the sweltering heat during a smuggling attempt.

Dozens of migrants were packed into the back of a truck found on the outskirts of San Antonio on June 27.

Two Americans have been indicted in federal court in that case and could face the death penalty if convicted. Two Mexicans have been charged with lesser crimes.

"We are heartbroken to learn of yet another tragic incident of migrants taking the dangerous journey,” Homeland Security Secretary Alejando Mayorkas said on Twitter, vowing to work with local investigators to find those responsible.

“Smugglers are callous and only care about making a profit,” Mayorkas said.

Uvalde Police Chief Daniel Rodriguez suspected the migrants were dehydrated due to the heat of the train cars during warm weather, KSAT television reported.

“It's sad to see that so many undocumented immigrants were found in this condition, and two of them lost their lives. It's heartbreaking,” KSAT quoted Rodriguez as saying.

The Union Pacific railroad would lead the investigation, Uvalde police said.

The Mexican consulate in Eagle Pass, Texas, said on Twitter it was aware of the incident and in communication with U.S. officials to determine if any of the victims were Mexican.

Homeland Security had yet to determine the nationalities of the victims or whether any families or children were among them, a department official with knowledge of the matter said, speaking on condition of anonymity.

Knippa is about 72 miles (115 km) west of San Antonio and about 120 miles (190 km) from the Mexican border.

It is near the town of Uvalde, which was the scene of a mass shooting at an elementary school in May last year that killed 19 school children and two teachers. (Reuters)

•Migrants are being rescued aftera truck headed for the US border was found abandoned in Mexico 2022. (AFP)

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