Officials in Uvlade, Texas, received an anonymous emergency call on Friday advising them that numerous immigrants were suffocating inside a train.

At least 15 immigrants needed immediate medical attention, police said.

US Border Patrol officials were notified and able to stop the train just east of Knippa, Texas, in Uvalde County.

Federal investigators were looking into the possibility of human smuggling, investigators with the US 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.

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 US 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 said.

Australian Associated Press