At least 46 people have been found dead inside of a truck in San Antonio, Texas.
The discovery is the most horrific incident of human smuggling over the US-Mexico border in recent times.
A San Antonio Fire Department official said there were no signs of water in the truck, which was found next to railway tracks in a remote area on the city's southern outskirts.
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Sixteen additional people found inside the trailer were taken to hospitals to be treated for heat stroke and exhaustion.
Of these, four were minors. However, officials said no children were killed in the incident.
"The patients that we saw were hot to the touch. They were suffering from heat stroke, exhaustion," San Antonio Fire Chief Charles Hood told a news conference.
"It was a refrigerated tractor-trailer, but there was no visible working AC unit on that rig."
Temperatures in San Antonio, about 250km from the Mexican border, reached a high of 39.4C on Monday, with high humidity.
The city's Police Chief William McManus said a person who works in a nearby building heard a cry for help and came out to investigate.
The worker found the trailer doors partially opened and looked inside to find a number of dead bodies.
Mr McManus said this was the largest incident of human smuggling in the city and said three people were in custody following the incident, though their involvement is not yet clear.
A spokesperson for US Immigration and Customs Enforcement (ICE) said that its Homeland Security Investigations division was investigating "an alleged human smuggling event" in coordination with local police.
Mexico's Foreign Minister Marcelo Ebrard called the incident a "tragedy in Texas" on Twitter and said consular officials would go to the hospitals where victims had been taken to help "however possible".
A spokesman for the Honduran foreign ministry said the country's consulates in Houston and Dallas would be investigating the incident.
Mr Ebrard said two Guatemalans were hospitalised.
Guatemala's foreign ministry said on Twitter that consular officials were going to the hospital "to verify if there are two Guatemalan minors there and what condition they are in".
The news comes after a similar incident in July 2017 that saw ten people die after being transported in a tractor-trailer that was discovered by San Antonio police in a Wal-Mart car park.
The driver, James Matthew Bradley, Jr, was sentenced to life in prison for his role in the smuggling operation the following year.
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