Federal agents are investigating the deaths of at least six people thought to be immigrants found inside a shipping container at a Union Pacific rail yard near the border with Mexico in Laredo, Texas, on Sunday as a “potential human smuggling event”.

Officials reportedly have also said the death of a seventh person whose body was found near railroad tracks outside San Antonio, Texas – 150 miles (241km) to the north – may be connected to the case.

A Union Pacific employee found the bodies of six people inside a shipping container Sunday afternoon, said Jose Baeza, the Laredo police department public information officer.

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 heatstroke.

“I’ve ruled that an accidental death,” she said, adding that she believes the others also died from heatstroke but could not rule on their cause of death until she completes their autopsies.

Stern estimates it took up to eight hours for the people to succumb to illness.

“Based on my examination on the scene and what I know of from the investigation, I really believe they were dead in less than eight hours,” Stern said.

Stern found identification cards and cellphones that indicate the deceased may be from Mexico and Honduras, but fingerprints were taken and shared with US 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.

Homeland Security Investigations said in a statement that it is “actively investigating this case as a potential human smuggling event with assistance from the Laredo police department and Texas Rangers.”

“This was a horrific scene,” Stern said, also noting that immigrant deaths are a common occurrence in the 10-county region her office covers. “This spring has been busier than it was this time last year,” the medical examiner said, referencing the number of migrant deaths recorded by her office last year.

As CBS News reported, Bexar county sheriff Javier Salazar – whose jurisdiction encompasses San Antonio – said investigators found the seventh dead person while looking into an alert that one of the containers had been opened over the weekend near the location of the body.

“On these trains, there is a censor on some of the containers that alerts whenever a container’s opened,” Salazar said to reporters, according to CBS. “Once those bodies were found in Laredo, they came back to this location here and started patrolling up and down the railroad tracks until such time that they found him.”

The container can’t be opened from within, according to Salazar.

Border encounters dropped toward the end of Joe Biden’s presidency and reached record low numbers under Donald Trump’s second administration. About 40 people were encountered daily in March crossing illegally by border patrol agents in Laredo, making it the third busiest sector among nine along the US-Mexico border, according to the agency’s statistics.

The travel history of the shipping container was not known, and the criminal investigation has not yet determined why the people who died did not climb out of the shipping container.

“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 US border with Mexico and a common nexus for the illegal movement of people.

On 28 June 2025, two smugglers were sentenced to life in prison for what remains the US’s deadliest human smuggling attempt across the US’s southern 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 US 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 the US.

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.

• Guardian staff contributed reporting