16 others, including four children, have been taken to hospital.
At least 46 people have been found dead in an abandoned lorry on the outskirts of San Antonio in Texas, in what has been labelled a "horrific, human tragedy" by the city's mayor.
Fire officials said they found "stacks of bodies" in the truck and confirmed that 16 people, including four children, had been taken to hospital.
Emergency services had arrived at the scene at about 6pm local time after reports of a dead body, San Antonio fire chief Charles Hood told reporters.
"We're not supposed to open up a truck and see stacks of bodies in there. None of us come to work imagining that," he said.
The survivors had apparently been "hot to the touch" and were suffering from heat stroke and heat exhaustion.
An official cause of death has not yet been recorded, but temperatures in the city reached a high of 39.4C on Monday,
Sky News reports.
Hood told the press there was "no visible working AC [air conditioning] unit" on the trailer.
Authorities have said they believe the people in the back of the lorry were migrants.
San Antonio is a city located 150 miles from the US border with Mexico and is a major transit route for people-smugglers, the
BBC reports.
The nationalities of those found in the lorry have not yet been established. It is also not yet known how long they had been abandoned on the side of the road.
Police Chief William McManus said three people had been taken into custody but it is unclear if they were definitively connected with the incident.
He also stated that the case has been turned over to US Homeland Security Investigations and is now a federal investigation.
Texas governor Greg Abbott blamed the deaths on president Joe Biden, saying they were a result of his "deadly open border policies."
Meanwhile, San Antonio Mayor Ron Nirenberg labelled it a "horrific, human tragedy", adding: "Our job is not to ask why. Our job is to ask how we can help."
