Three people were taken into custody, but it was unclear if they were absolutely connected with human trafficking, McManus said.
Those in the trailer were part of a presumed migrant smuggling attempt into the United States, and the investigation was being led by U.S. Homeland Security Investigations, McManus said.
Those in the trailer were in a presumed migrant smuggling attempt in South Texas, according to an official who spoke to The Associated Press on condition of anonymity because the information had not been authorized for public release.
It may be the deadliest tragedy among thousands who have died attempting to cross the U.S. border from Mexico in recent decades. Ten migrants died in 2017 after being trapped inside a truck that was parked at a Walmart in San Antonio. In 2003, 19 migrants were found in a sweltering truck southeast of San Antonio.
Big rigs emerged as a popular smuggling method in the early 1990s amid a surge in U.S. border enforcement in San Diego and El Paso, Texas, which were then the busiest corridors for illegal crossings.
Before that, people paid small fees to mom-and-pop operators to get them across a largely unguarded border. As crossing became exponentially more difficult after the 2001 terror attacks in the U.S., migrants were led through more dangerous terrain and paid thousands of dollars more.
Heat poses a serious danger, particularly when temperatures can rise severely inside vehicles. Weather in the San Antonio area was mostly cloudy Monday, but temperatures approached 100 degrees.