Six people were found dead on Sunday afternoon in a boxcar in Laredo, Texas, a spokesman for the Laredo Police Department said.

At approximately 3 p.m. local time, an employee for Union Pacific who was responsible for loading and unloading train cars at a rail yard before they headed north reported the discovery of the bodies, said the spokesman, Investigator Joe E. Baeza.

The police and fire departments later confirmed the deaths, he added.

The circumstances of their deaths were unknown, he said, adding that the police were investigating. The identities of the people were not known on Sunday.

“Union Pacific is saddened by this incident and is working closely with law enforcement to investigate,” Daryl Bjoraas, a spokesman for Union Pacific, said in a statement.

The city of Laredo is on the U.S.-Mexico border, about 160 miles southwest of San Antonio.

In the last decade, there have been several incidents where people have been found dead in train cars or trucks in or near U.S. border cities. Many of those victims were migrants, who were unaware that temperatures exponentially rise inside those kinds of containers.

High temperatures in Laredo were more than 90 degrees on Sunday.

Officials could not confirm if those who died were migrants trying to cross into the United States.

In 2022, 53 migrants — 47 adults and six children — were found in a tractor-trailer on the outskirts of San Antonio in what was one of the deadliest migrant smuggling incidents in the nation’s history.

A year later, the bodies of two people believed to be migrants, along with five others who were in critical condition, were found inside a shipping container on a train in Uvalde County, Texas.

This is a developing story. Check back for updates.