At least two migrants died inside a train car near Knippa, Texas, in an incident that is being investigated as a suspected human smuggling operation, local authorities and a senior U.S. official said Friday.

More than a dozen people were found inside train cars on Friday afternoon after someone alerted 911 that migrants were “suffocating” there, according to the police department in neighboring Uvalde. U.S. Border Patrol agents responded to the scene, alongside local and Texas law enforcement authorities.

Five migrants were flown to hospitals in the San Antonio area, and another five were transported to local hospitals, the Uvalde Police Department said in a statement. Their conditions remained “unknown” as of Friday evening, Uvalde police added.

The train was bound from Eagle Pass to San Antonio, Union Pacific told CBS News in a statement. Union Pacific said it was alerted by Border Patrol at about 4 p.m. local time that 15 people had been found in two different rail cars. 

A senior U.S. official said all indications suggest the victims were migrants being smuggled into the country illegally. Migrants are often transported inside train cars, tractor trailers and other vehicles by human smugglers after crossing the U.S.-Mexico border without authorization.

Migrant smuggling is often dangerous and in some cases, deadly. In June 2022, 53 migrants, including unaccompanied teenagers, lost their lives inside a sweltering tractor-trailer that was abandoned outside of San Antonio — the deadliest migrant smuggling case in U.S. history.

In a statement Friday, Secretary of Homeland Security Alejandro Mayorkas said he was “heartbroken to learn of yet another tragic incident of migrants taking the dangerous journey.” He said federal Homeland Security Investigation (HSI) agents were probing the case.

“We will work with the Uvalde County Sheriff’s Office to hold those responsible,” Mayorkas added. “Smugglers are callous and only care about making a profit.”