A man who was found dead Monday by the railroad tracks southwest of San Antonio, Texas, is believed to be connected to the six people who likely died of heat stroke inside a shipping container near the Mexico border, according to authorities. 

The body of the seventh person was located nearly 150 miles north of a Union Pacific rail yard in Laredo, where the six bodies were discovered on Sunday afternoon, Bexar County Sheriff Javier Salazar said Monday. U.S. Immigration and Customs Enforcement told CBS News in a statement that the agency is investigating the deaths inside the boxcar as a potential human smuggling incident.

Salazar said investigators found the seventh person after there was an alert that one of the containers had been opened at some point over the weekend near the location.

"On these trains, there is a censor on some of the containers that alerts whenever a container's opened," Salazar told reporters. "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." Salazar said the container cannot be opened from the inside.

The sheriff said the train that contained the body found Monday was from Del Rio and had split up near San Antonio, with one half going to Laredo and the other to Houston, suggesting it's possible there might have been more people in the container at some point.

"At this point, we don't know if it was opened to let people that made it out successfully or they just opened it to dump the body," Salazar said. 

The person found Monday was identified as a man carrying Mexican identification, according to the sheriff.

Salazar said San Antonio police received a report Saturday from a relative of one of the people inside the container. The relative, who lives in another state, told police in a phone call that they received a message saying "it was getting very, very hot and they were having some physical trouble as a result of it," Salazar said.

The sheriff said the person who sent the message is believed to be among the six who were found dead.

Webb County Medical Examiner Dr. Corinne Stern, who is conducting autopsies, said she believes the individuals originated from Mexico and Honduras. An autopsy completed for a 29-year-old Mexican woman revealed she died of hyperthermia, or heat stroke. 

"I've ruled that an accidental death," Stern 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."

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

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