Six people were found dead Sunday in a Union Pacific Railroad boxcar in Laredo, Texas, a city on the Mexican border, police there said.

A railroad employee found the bodies, police said, adding that, “This is an on going fluid investigation.”

Union Pacific said in a statement that it was “saddened by this incident and is working closely with law enforcement to investigate.” 

The people weren’t publicly identified and authorities didn’t immediately give any more information about the probe.

Temperatures reached 97 degrees in Laredo on Sunday afternoon, which means it probably felt hotter than 100 degrees in the boxcar, according to CBS San Antonio affiliate KENS-TV.

Laredo is some 157 miles southwest of San Antonio.