At least 46 migrants have been found dead inside a tractor-trailer vehicle in San Antonio, Texas.
Another 16 people - including four children - who needed medical attention were taken to nearby hospitals, San Antonio Fire Chief Charles Hood told reporters.
"We're not supposed to open up a truck and see stacks of bodies in there," he said.
Hood said he was hopeful that all 16 people taken to hospital would survive.
All of them were hot to the touch and appeared to be suffering from heat stroke. There were no signs of water or working air conditioning in the truck.
Local media reported the truck was involved in a suspected human smuggling incident.
San Antonio is about 240km from the border with Mexico.
San Antonio Police Chief William P. McManus said three people had been taken into custody.
The survivors lacked water and air conditioning. "The patients that we saw were hot to the touch," he said.
The grim discovery was made at 5.50pm on Monday evening (local time) in an undeveloped area of southwest San Antonio near the railroad tracks.
A person who worked in the area reported hearing a cry for help and spotted at least one body, officials said.
That person found the trailer with the doors partially opened and noticed multiple dead bodies inside.
The 18-wheeler truck was found near Lackland Air Force Base.
Several San Antonio police vehicles, and fire trucks and ambulances, can be seen in Quintana Rd on the Southwest Side of the city.
US Border Patrol officers are also at the scene.
Texas congressman Tony Gonzales said it was 100 degrees Fahrenheit (38 degrees Celsius) in the area today.
San Antonio has previously been the scene of mass people smuggling deaths.
Today's tragedy may be the deadliest tragedy among thousands who have died attempting to cross the US 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 US 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 mother-and-father operators to get them across a largely unguarded border. As crossing became exponentially more difficult after the 2001 terror attacks in the US, 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 today, but temperatures approached 100 degrees.
Just last week the Rio Grande Valley Sector Border Patrol said it had disrupted four human smuggling events, resulting in 50 arrests.
- Additional reporting: AP