At least 46 people have been found dead in a abandoned truck on the outskirts of San Antonio, Texas.
A fire official said 16 people, including four children, were taken to hospital.
Survivors were “extremely hot” and suffered from heat and exhaustion.
San Antonio, located 250 kilometers (150 miles) from the US-Mexico border, is a highway for human traffickers.
Human traffickers often use trucks to transport undocumented immigrants after meeting them in remote areas where they have been able to cross the United States.
“They had families … and they were trying to find a better life,” said San Antonio Mayor Ron Nirenberg. “It’s not a bad thing, it’s a human tragedy.”
Officials say the city worker found the scene after hearing people crying for help.
Paramedics first arrived at the scene around 6pm local time (23:00 GMT), San Antonio fire chief Charles Hood told reporters.
“We should not open the truck and see a lot of corpses among us. None of us come to work thinking that,” he said.
He said the car left by the driver had no active air conditioners and no drinking water inside.
San Antonio weather is extremely hot in the summer months, with temperatures reaching 39.4C (103F) on Monday, and it is suspected that the victims may have died from fever and dehydration.
Mexico’s Foreign Minister Marcelo Ebrard said two Guatemalan people were among those taken to hospital. The other victims were not immediately identified.
Three people have been remanded in custody and an investigation has been launched into the allegations against the federal agents.
Mexican Attorney General Rubén Minutti was sent to the scene and the San Antonio ambassador said he would provide “full support” if Mexican citizens were among the dead.
Edward Reyna, a security guard in the yard of the yard, said he was not surprised to come to work at night to hear the news. He said he missed the times when he saw foreigners jumping off a train passing near where the truck was found.
“I thought that soon, someone would be injured,” said Mr Reyna. “The carts that bring them don’t care about them.”
This story has been played in San Antonio before, but not on this page. In 2017, 10 immigrants were found dead inside the same tractor trailer outside Walmart – also south of the city.
The southern part of San Antonio is a tunnel with two main roads connecting cities and towns along the Texas border.
Many rural communities, a few yards and a handful of developing areas in this part of San Antonio make it easy for a four-sided truck to be invisible – even impossible.