Authorities in Texas have launched an investigation after a truck with 46 dead migrants in its trailer was discovered just 200 kilometres from the border with Mexico.
A warehouse worker on the outskirts of San Antonio heard cries for help in the early evening on Monday (local time) and raised the alarm after peering inside the back of the semi-trailer to find dozens of bodies.
The trucks have been used for human trafficking operations for many years, but the enormous loss of life makes Tuesday's discovery the deadliest in recent history.
In 2003, police discovered 70 undocumented migrants trapped inside a sweltering truck trailer in Victoria, Texas. Nineteen of them died.
At a press conference on Tuesday afternoon, San Antonio Police Department's William McManus said three people were in custody.
A federal investigation had been opened by Homeland Security, and sniffer dogs had already been deployed to try and track anyone else who might have been with the vehicle.
Forty firefighters and 10 trucks arrived within minutes of the emergency call.
Fire chief Charles Hood says 12 adults and four children were taken to hospital, all conscious.
"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," Hood said.
"It was a refrigerated tractor trailer, but there was no visible working [air conditioning] unit on that rig."
Local charities and the mayor also rushed to the scene.
"It's tragic," said San Antonio Mayor Ron Nirenberg.
"Forty-six individuals who are no longer with us. Who had families. Who were trying to find a better life... This is nothing short of an horrific human tragedy."
Migrant numbers have surged under the Biden administration, but there's been little change in policy at the border, or the numbers being let through.
Still, Texas governor Greg Abbott, a hardline Republican and supporter of former President Donald Trump, blamed Joe Biden for the deaths.
"They are a result of his deadly open border policies," he tweeted.
Abbott has previously claimed to have solved border migration issues while the Biden administration has been in power, by spending nearly $5 billion (NZD) turning back tens of thousands of migrants, and "apprehending" 265,000 more.
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