The death toll from a human trafficking incident on Monday in which migrants suffered under extreme Texas heat inside a truck rose to 50, US and Mexican officials said on Tuesday, in one of the most fatal incidents of human smuggling along the US-Mexico border in recent history.

The migrants died inside a tractor-trailer in San Antonio, Texas, where temperatures swelled to a high of 103 degrees Fahrenheit (39.4 degrees Celsius).

Local authorities responded on Monday afternoon to a remote area in the city’s southern outskirts and found the truck parked next to railroad tracks. Bodies were strewn over a couple of blocks after it is believed that the back door of the trailer was opened, a local law enforcement official told Reuters on Tuesday.

Local and US authorities said there were no signs of water and no visible working air conditioning inside the truck.

"It's unspeakable," San Antonio Mayor Ron Nirenberg said on MSNBC, noting that his community depends on migrants while there is a labor shortage. "It's a tragedy beyond explanation."

US President Joe Biden in a statement on Tuesday called the incident "horrifying and heartbreaking."

Calling out the multibillion-dollar criminal smuggling industry, Biden said that "exploiting vulnerable individuals for profit is shameful," and said that his administration was working to crack down on these networks. Biden has struggled with a record number of migrant crossings at the US-Mexico border since he took office in January 2021.

Some 22 Mexicans, seven Guatemalans, and two Hondurans were identified among the dead, Mexico Foreign Minister Marcelo Ebrard said on Twitter on Tuesday. There was no information on the nationality of the other 19, Mexican officials said.

More than a dozen people were transported to hospitals for heat stroke and exhaustion, including four minors, but no children were among the dead, the San Antonio Fire Department said.

The Baptist Medical Center in San Antonio on Tuesday said that three of the five patients brought there had died.

The truck may have been carrying around 100 migrants, but the exact number remains unclear, according to a law enforcement official and US Customs and Border Protection (CBP) official briefed on the investigation.

The migrants were sprinkled in a pungent substance, said the two sources, speaking on condition of anonymity. In past incidents, smugglers have used different substances to disguise the smell of human cargo and evade canine detection.

A spokesman for the Honduran foreign ministry told Reuters that the country's consulates in Houston and Dallas would be investigating the incident.

Mexican officials are assisting the families of the victims and the transfer of the bodies, said Mexican President Andres Manuel Lopez Obrador, who offered condolences "to the relatives of this catastrophe." Ebrard on Tuesday said Mexico was opening an investigation into the tragedy.

The city's police chief, William McManus, on Monday said a person who works in a nearby building heard a cry for help and came out to investigate. The worker found the trailer doors partially opened, looked inside, and found a number of dead bodies.

It appeared that the migrants had recently crossed the US-Mexico border and were picked up by the truck to be taken to where they would work, a Mexican official said.

US Immigration and Customs Enforcement (ICE) confirmed the death toll and said that its Homeland Security Investigations division had detained three individuals "believed to be part of the smuggling conspiracy."

The local law enforcement official said one suspect was arrested by the truck and two were found at a house after police tracked a vehicle an eyewitness described leaving the scene.

RECORD CROSSINGS

The deaths once again highlight the challenge of controlling migrant crossings for Biden, a Democrat who came into office pledging to reverse some of the hardline immigration policies of his Republican predecessor, Donald Trump. Republicans have criticized Biden's border strategy ahead of the midterm congressional elections in November.

Lopez Obrador on Tuesday said he will meet with Biden in Washington on July 12 to discuss migration.

The I-35 highway near where the truck was found runs through San Antonio from the Mexican border and is a popular smuggling corridor because of the large volume of truck traffic, according to Jack Staton, a former senior official with ICE's investigative unit who retired in December.

In July 2017, 10 migrants died after being transported in a tractor-trailer that was discovered by San Antonio police in a Wal-Mart parking lot. The driver, James Matthew Bradley, Jr., was sentenced the following year to life in prison for his role in the smuggling operation.

A spokesperson for the United Nations High Commissioner for Human Rights Michelle Bachelet called Monday's incident "distressing," adding that it illustrates again the need for safe pathways for migration and accountability for those involved in human smuggling.

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