At least 50 migrants died after being stuck inside a sweltering semi-trailer in San Antonio, U.S. and Mexican officials said on Tuesday, in one of the deadliest human trafficking incidents in recent history.

The migrants were discovered dead inside the truck's trailer on the outskirts of the Texas city on Monday, where temperatures swelled to a high of 103 Fahrenheit (39.4 Celsius).

Authorities responded to a remote area in the south part of the city 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 U.S. 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."

U.S. President Joe Biden said in a Tuesday statement that the incident was "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 U.S.-Mexico border since he took office in January 2021.

INVESTIGATIONS UNDERWAY

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 said on Tuesday 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 U.S. 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.

The surviving migrants will likely be released into the United States to pursue asylum or other forms of humanitarian relief, the CBP official and two other law enforcement officials told Reuters.

Federal prosecutors are expected to open a criminal investigation into the matter, one of the law enforcement sources said. The U.S. Attorney's Office for the Western District of Texas declined to comment.

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

Mexican President Andres Manuel Lopez Obrador offered condolences "to the relatives of this catastrophe." Ebrard on Tuesday said Mexico was opening an investigation.

San Antonio'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 U.S.-Mexico border and were picked up by the truck to be taken to where they would work, said a Mexican official and two of the U.S. officials.

U.S. 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. A Mexican official, speaking on condition of anonymity, said at least one of the three people arrested had American citizenship.

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IMMIGRATION CHALLENGES

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 hard-line 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.

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.

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.

"It was only a matter of time before a tragedy like this was going to happen again," Staton said.

(GRAPHIC-Border crossings encounters along U.S.-Mexico border:

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