The six people were from Honduras and Mexico and included a 14-year-old boy, all part of a human smuggling effort on a freight train, authorities said.
Police released more details about the discovery made Sunday in Laredo, near the U.S. border with Mexico, but said federal authorities were leading the investigation.
The six people were put in the shipping container on Saturday in Del Rio, Texas, two days after the train departed from Long Beach, California, Laredo Police Chief Miguel Rodriguez Jr. said.
He said the train traveled to the San Antonio area from Del Rio before arriving Sunday in Laredo. Laredo is a busy land port for trade on the U.S.-Mexico border and a common nexus for the illegal movement of people.
Law enforcement says five people whose remains were discovered in the boxcar died from heatstroke and another died from hyperthermia.
A seventh body was discovered along railroad tracks Monday in Bexar County, about 150 miles north of Laredo, according to authorities.
Rodriguez said he could not confirm if the seventh person found dead is connected to the victims found on the train.
Smuggling on trains 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 enters the U.S.
The Associated Press contributed to this report.