The driver appears to have run off leaving the door ajar but survivors were apparently too weak to stagger out into the fresh air, as Correspondent Ian Woods reports
Fifty people have died after being found in the back of an abandoned, sweltering lorry in San Antonio, Texas.
A total of 46 people were found dead at the scene on Monday said officials. Another 16 people were hospitalised, including four children, while four died later on Tuesday in hospital.
It is among one of the worst tragedies to claim the lives of thousands of people attempting to cross the US border from Mexico in recent decades.
President Joe Biden called the deaths “horrifying and heartbreaking” and said initial reports were that smugglers or human traffickers were to blame.
According to Roberto Velasco Álvarez, head of the North America department in Mexico’s Foreign Relations Department, 22 of the dead were from Mexico, seven from Guatemala and two from Honduras.
The home countries of the other 19 have not yet been verified, he said.
Among the dead were 39 males and 11 females, said Police Chief William McManus.
San Antonio Mayor Ron Nirenberg said the initial 46 who died had “families who were likely trying to find a better life".
“This is nothing short of a horrific human tragedy,” Mr Nirenberg said.
Following the tragedy, President Biden vowed to crack down on human smugglers and traffickers.
He said in a statement: “Exploiting vulnerable individuals for profit is shameful, as is political grandstanding around tragedy, and my administration will continue to do everything possible to stop human smugglers and traffickers from taking advantage of people who are seeking to enter the United States between ports of entry."
But Texas governor Greg Abbott, a Republican running for re-election, tweeted: "These deaths are on Biden.
"They are a result of his deadly open border policies. They show the deadly consequences of his refusal to enforce the law."
Aaron Reichlin-Melnick, policy director at the American Immigration Council, wrote that he had been dreading such a tragedy for months.
“With the border shut as tightly as it is today for migrants from Mexico, Guatemala, Honduras and El Salvador, people have been pushed into more and more dangerous routes. Truck smuggling is a way up,” he tweeted.
A city worker heard a cry for help from the truck shortly on a back road shortly before 6pm local time on Monday and discovered the gruesome scene, Chief McManus said.
Hours later, body bags lay spread on the ground near the trailer as a grim symbol of the tragedy.
How long the lorry was and those in it were abandoned on the side of the road was not immediately known.
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The patients were hot to the touch and dehydrated, and no water was found in the trailer, he said.
“They were suffering from heat stroke and exhaustion," Mr Hood said. "It was a refrigerated tractor-trailer, but there was no visible working AC unit on that rig.”
Weather in the San Antonio area was mostly cloudy on Monday, but temperatures approached 38C (100F).
Those in the trailer were part of a presumed migrant smuggling attempt into the United States, and the investigation was being led by US Homeland Security Investigations, Mr McManus said.
Three people were taken into custody, but it was unclear if they were definitively connected with human trafficking, he added.
US Customs and Border Protection reported 557 deaths on the southwest border in the 12-month period ending on September 30, more than double the 247 deaths reported in the previous year and the highest since it began keeping track in 1998. Most are related to heat exposure.
In 2017, 10 migrants died 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.
In England, 39 Vietnamese migrants were found dead in a truck trailer in Essex in 2019. Four men were jailed for manslaughter.