At least 46 migrants were found dead Monday in and around a tractor-trailer that was abandoned on the side of a road in San Antonio, in central Texas, authorities said.
The grim discovery was one of the worst disasters involving migrants in the United States in recent years -- and came five years after a similar deadly incident in the same city, a few hours from the Mexican border.
"At this time we have processed approximately 46 bodies that have been triaged and tagged and declared deceased," San Antonio Fire Chief Charles Hood told reporters.
He said that 16 people had been transported to hospital alive and conscious - 12 adults and four pediatric patients.
"The patients that we saw were hot to the touch, they were suffering from heat stroke, heat exhaustion, no signs of water in the vehicle, it was a refrigerated tractor-trailer but there was no visible working AC unit on that rig," Hood said.
Officials reported three people were in custody over the incident.
San Antonio, which lies some 250 kilometers (150 miles) from the border, is a major transit route for people smugglers.
The vehicle was found on a road near Highway I-35, a major US artery that stretches all the way to the border with Mexico.
A massive emergency operation was underway at the scene involving police, firefighters and ambulances.
Texas Governor Greg Abbott hit, a Republican who advocates a tough line on immigration, hit out at President Joe Biden over the disaster -- blaming his "deadly open border policies."
"These deaths are on Biden," Abbott tweeted. "They show the deadly consequences of his refusal to enforce the law."
San Antonio was the site of a similar migrant tragedy in 2017, when 10 people suffocated to death in a sweltering trailer with broken air conditioning and clogged ventilation holes as they traveled into the United States.
Dozens more had been hospitalized with heat stroke and dehydration – with the truck believed to have been holding as many as 200 people, most of whom fled when it stopped in a parking lot.
"Lord have mercy on them. They hoped for a better life," tweeted Gustavo Garcia-Siller, the archbishop of San Antonio after news broke of Monday's discovery.
"Once again, the lack of courage to deal with immigration reform is killing and destroying lives."