Six people, including a teenager, were found dead inside a shipping container at a Texas rail yard near the U.S.-Mexico border, officials said.
A seventh person who was found dead along train tracks in an area outside San Antonio is also believed to have been part of the same group in what is a suspected smuggling incident, authorities said.
The six bodies were discovered Sunday at the Union Pacific rail yard in Laredo, police said. An employee at the rail yard called police after discovering the bodies during a routine rail car inspection, police said.
The victims include a 14-year-old boy and a 24-year-old man from Honduras, as well as a 29-year-old woman and two men -- aged 45 and 56 -- from Mexico, according to the Webb County Medical Examiner's Office.
So far, the woman has been confirmed to have died from hyperthermia, according to the Webb County Medical Examiner's Office, which said it is "highly probable that hyperthermia was the cause of death for the entire group."
The body of the seventh person was found Monday afternoon near tracks in Bexar County, some 150 miles north of Laredo, according to authorities. The man, whose identity has not yet been confirmed, was carrying a Mexican voter registration card, according to Bexar County Sheriff Javier Salazar.
"At this point, the prevailing theory is that he's a resident of Mexico that was among that group that was being smuggled into the country in one of these shipping containers," Salazar said during a press briefing on Monday.
It is unclear if the man had died while in the shipping container and his body was dumped, or if he died in a fall from the train, he said, noting that the medical examiner will be determining the cause and manner of death.
Salazar said the shipping containers can only be opened from the outside, and that sensors go off when they are opened.
He said the train is believed to have originated in Del Rio, Texas, where the sensor did go off, presumably to load people on. The sensor went off again near where the body of the seventh person was found in Bexar County, he said.
"The fact that a sensor hit from here indicates someone opened that from outside," Salazar said. "Our belief at this point is that it was most likely smugglers, coyotes that opened it from the outside."
It is unclear if there were more people on the train who were successfully let out at that point, he said.
Salazar said the train continued on and was split up at a station, with half of it going to Houston and the other half to Laredo, where the six other people were found dead.
One of the people found dead in Laredo is believed to have contacted a relative on Saturday from inside the shipping container, saying in a message that "it was getting very, very hot, and that they were having some physical trouble as a result of it," Salazar said.
The relative, who lives in a different state, contacted police, and San Antonio officers were dispatched to a location several miles from where the body was found in Macdona and did not find anything, he said.
The Webb County Medical Examiner's Office said in a statement Tuesday it is "working in close coordination with the Mexican Consulate to facilitate communication with the families of the deceased, ensure positive identification, and assist in the repatriation process as efficiently as possible."
Homeland Security Investigations and Texas Rangers are also investigating the incident, according to U.S. Customs and Border Protection.
Union Pacific said it is "saddened by this incident and is working closely with law enforcement to investigate."
ABC News' Laura Romero contributed to this report.