All six people whose remains were discovered in a boxcar in South Texas died from heat stroke before they were found by law enforcement, Laredo mayor Dr. Victor Treviño said during a news conference Thursday.

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The victims were previously identified as five males and one female, ranging in age from 14 to 56, according to the Webb County Medical Examiner’s Office. Authorities have not released their names.

Three of the people were from Mexico and three were from Honduras, police said.

“Today, our community is still mourning a heartbreaking discovery,” Treviño told reporters.

“Let me be clear, we are demanding justice for these lives lost,” he went on. “It doesn’t matter where they came from.”

The Laredo Police Department was called to a Union Pacific train on Sunday afternoon after an employee reported a “trailer box car with the discovery of multiple casualties in the car.”

The train had departed from Long Beach, Calif., on May 7 and arrived in Del Rio, Texas, on May 9, Laredo Police Chief Miguel Rodriguez said. It was there that the six people were loaded into the boxcar, according to the preliminary investigation.

The train traveled from Del Rio through San Antonio and onward to Laredo. Local police said they presume the deaths were tied to a human smuggling operation, and have contacted the Department of Homeland Security. Federal officials are leading the investigation, Rodriquez said.

A seventh person was found dead near railroad tracks in San Antonio on Monday. That investigation remains ongoing. Officials have not determined whether it is related to the Laredo case, Rodriquez said.

Treviño, a local physician, urged people who might be considering crossing into the United States to avoid the perilous journey and instead wait for legal authorization.

“We understand the desire for a better life, but do not come here illegally,” he said, before repeating the message in Spanish.

“Smugglers do not care about your safety and will place you in a life threatening situation or condition without hesitation,” he went on. “No opportunity to avoid your life urge you to seek legal and safe pathways, protect your life, protect your future.”