AT least 53 migrants have been found dead in the back of a truck that reached an estimated 150F as cops discovered stacks of bodies following desperate cries for help.
Another 16 people - including four children - were rushed to a hospital in a critical condition following the horrifying discovery in San Antonio, Texas.
Officials announced on Wednesday that the death toll has risen to 53 people, which include 22 migrants from Mexico, seven from Guatemala and two from Honduras.
The migrant bodies were found inside an abandoned tractor-trailer, officials said. The victims include 39 men and 12 women.
The horror discovery was made after a worker heard a cry for help - leading to "" being found inside the vehicle, the San Antonio Police Department said.
San Antonio has recorded temperatures of 100F (37.8C) or more on 17 days this month.
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It's estimated that the temperature inside the vehicle was around 150F (65C).
Three suspects were taken into custody following the deadly discovery, but it is unclear if they are "absolutely connected to this or not," SAPD Chief William McManus said.
Police said they found 16 victims - 12 adults and four kids - conscious in the back of the truck and rushed them to local hospitals.
It's understood the survivors are "fighting for their lives" in hospital, suffering from severe heat stroke and exhaustion.
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"The patients that we saw were hot to the touch... no signs of water in the vehicle or visible working AC inside," the city’s fire chief Charles Hood said.
"None of these people were able to extricate themselves out of the truck, so they were still in there awaiting help when we arrived. Too weak in state to actually get out and help themselves."
San Antonio Mayor Ron Nirenberg said the gruesome find was a "horrific human tragedy".
"There are, that we know of, 46 individuals who are no longer with us, who had families, who were likely trying to find a better life," he said.
"And we have 16 folks who are fighting for their lives in the hospital."
Images from the scene showed roads blocked and dozens of first responders surrounding the now-abandoned truck next to railroad tracks.
US Border Patrol, police, the San Antonio fire department and ambulances are at the scene.
Officials have yet to confirm the cause of the 50 that perished.
According to the Washington Post, the truck was registered under the name of an Alamo man.
However, the man's son-in-law, Isaac Limon, told the outlet the number on the truck had been fraudulently used by smugglers.
"It was a perfect setup," he told the outlet, adding that his father-in-law was shaken up by the incident.
"The truck is here. I’m looking at it right now. Sad to say, but he’s a bit of a victim, too, because people believe it was him."
Local resident Ruby Chavez, 53, told the New York Times the area where the bodies were discovered is a well-known "drop-off spot" for migrants.
"You can tell they just get here. We see them with backpacks or asking for food or money," she told the paper.
'TOO MANY LIVES HAVE BEEN LOST'
Homeland Security Secretary Alejandro Mayorkas took to hours later, calling the discovery a "tragic loss of life".
He wrote: "I am heartbroken by the tragic loss of life today and am praying for those still fighting for their lives.
"Far too many lives have been lost as individuals – including families, women, and children – take this dangerous journey."
Mayorkas said Homeland Security had started an investigation with the help of San Antonio Texas Police and U.S. Customs and Border Protection.
"Human smugglers are callous individuals who have no regard for the vulnerable people they exploit and endanger in order to make a profit," he concluded.
"We will work alongside our partners to hold those responsible for this tragedy accountable and continue to take action to disrupt smuggling networks."
Joaquin Castro, who represents Texas' 20th congressional district, said: "This horrible tragedy is a reminder that we need a safe, orderly way for people to claim asylum."
'A HORRIBLE TRAGEDY'
An investigation was underway at the scene after an 18-wheeler was discovered with undocumented migrants inside, explains.
SAPD officers were searching the nearby surroundings for the truck's driver, who appeared to abandon the tractor-trailer before authorities located it, according to .
KSAT's shared an image showing San Antonio Fire Department's Mass Casualty Evacuation Ambulance nearby the semi-truck.
"We spoke with neighbors in the area," Waldman said in another tweet.
"They’re shocked and horrified, telling us that people come here for a better life and to work and this is the reality they’re met with."
Up to 20 emergency vehicles have converged at the scene, carrying 20 firefighters and 10 medical units, according to the .
Border patrol officers arrived at the scene, which was located alongside railroad tracks, at around 8pm on Monday.
Speaking to CBS affiliate , Antonio Fernandez, CEO of Catholic Charities, said: "We know that a lot of people have been taken to the hospitals.
"We're just trying to collect as much information as possible and see how we can help the people who survived.
"I'm not really sure those people are going to need a place to sleep tonight, or what is going to happen. We're just trying to find out as much information as we can help them."
It is one of the deadliest incidents involving migrants attempting to cross into the US from Mexico in recent decades.
In 2017, 10 migrants died after being trapped inside a parked truck at a Walmart in San Antonio.
In a similar situation in 2003, 19 migrants were found in a heated truck southeast of San Antonio.
Some 557 people have died on the southwest border in the 12 months leading up to September 30 - more than double the 247 deaths reported last year.
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It's the highest figure since record-keeping began in 1998, according to US Customs and Border Protection (CBP).
Texas governor Greg Abbott blamed the tragedy on the Biden administration, claiming "open borders" led to the horror event.