LAREDO, Texas (AP) — A South Texas medical examiner believes heat stroke may have led to the death of six people thought to be immigrants who were found Sunday afternoon inside a shipping container at a Union Pacific rail yard near the Mexico border in Laredo, Texas.

The people were found Sunday as workers were inspecting one of the containers, said Jose Baeza, a spokesperson for the Laredo Police Department.

Police and fire crews arrived at the scene shortly afterward. They confirmed that there were six people dead, five men and one woman, Baeza told reporters.

Dr. Corinne Stern, the Webb County Medical Examiner, is conducting autopsies and completed one for a 29-year-old Mexican woman who died of hyperthermia, or heat stroke. “I’ve ruled that an accidental death,” she said. “I believe that the remaining individuals probably all succumbed to heat stroke as well, but their exams are not completed at this time, so I will not rule on their cause and manner yet.”

Stern found identification cards and cellphones that indicate the deceased may be from Mexico and Honduras, but fingerprints were taken and shared with U.S. Border Patrol to help confirm their identities and nationalities through the Missing Alien Program.

The medical examiner's office also contacted the Mexican consulate after identifying the woman.

“This was a horrific scene,” Stern added, also noting that immigrant deaths is a common occurrence in the ten-county region her office covers. “This spring has been busier than it was this time last year,” the medical examiner said.

Border encounters dropped toward the end of the Biden administration and reached record low numbers under the second Trump administration. About 40 people are encountered crossing illegally by Border Patrol agents in Laredo in March making it the third busiest sector among nine along the southwest border, according to the agency's statistics.

The travel history of the shipping container was not known.

“Union Pacific is saddened by this incident and is working closely with law enforcement to investigate,” the rail company said in a statement.

Laredo is a busy land port of entry for trade on the U.S.-Mexico border and a common nexus for the illegal movement of people, although authorities have not said whether the six deaths were related to a smuggling operation.

Last year, two smugglers were sentenced to life in prison for what remains the nation’s deadliest human smuggling attempt across the U.S.-Mexico border. They were convicted in connection with the deaths of 53 migrants found in the back of a sweltering tractor-trailer in Texas in 2022.

Smuggling on trains crossing the border has long been a concern partly because trains headed to the United States often slow or stop in Mexico before crossing the border. That creates an opportunity for smugglers or immigrants to climb aboard or hide drugs or other contraband on a train before it crosses into America.

Union Pacific has worked with authorities for years to address drug smuggling and trespassers trying to cross the border on trains. As part of that effort, the railroad has installed inspection portals that scan the trains and take pictures to help spot any abnormalities that would suggest contraband or immigrants aboard the train.

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This story updates throughout to change reference of boxcar to shipping container, as per Union Pacific.