At least 51 migrants have died after being trapped inside a sweltering tractor-trailer truck found abandoned in Texas, authorities said on Tuesday, as two Mexican nationals tied to the smuggling incident were charged in U.S. federal court.

The deceased migrants, 39 men and 12 women, were discovered on Monday on the outskirts of San Antonio, Texas, in one of the deadliest human trafficking tragedies in recent history.

Two suspects identified as Juan Francisco D'Luna-Bilbao and Juan Claudio D'Luna-Mendez, both Mexican citizens, have been charged with possessing firearms while residing in the United States illegally, according to court documents and U.S. authorities.

Investigators traced the truck's vehicle registration to a San Antonio address that they placed under surveillance, and arrested the two men separately when each was seen leaving the residence.

A third suspect, described as a U.S. citizen who drove the truck, has also been taken into custody and was expected to be charged, but he remained hospitalized as of Tuesday evening, according to a Mexican official.

The truck had been left parked next to some railroad tracks on the outskirts of San Antonio, where temperatures soared to a high of 103 Fahrenheit (39.4 Celsius). Bodies were found inside the vehicle and strewn over a couple of blocks, after the rear door of the trailer had been opened, a local law enforcement official told Reuters on Tuesday.

Local authorities said there were no signs of water or visible means of air-conditioning inside the truck. The officials said there were "stacks of bodies" and that some of the migrants were hot to the touch.

"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 the incident was "horrifying and heartbreaking."

"Exploiting vulnerable individuals for profit is shameful," Biden said, vowing to crack down on multibillion-dollar criminal smuggling enterprises that have helped fuel a record number of migrant crossings at the U.S.-Mexico border since he took office in January 2021.

INVESTIGATIONS UNDERWAY

At least 27 Mexicans, three Guatemalans and four Hondurans were believed to be among the dead, according to officials from all three countries. There was no immediate information on the nationality of the other victims.

More than a dozen survivors were transported to area hospitals for heat stroke and exhaustion, including four minors.

At a press conference outside one of the hospitals treating patients on Tuesday, Rebeca Clay-Flores, a Democratic local official in Bexar County, Texas, sharply criticized Republican Governor Greg Abbott for a tweet on Monday night that blamed Biden's border policies for the incident even as the emergency response unfolded.

"While bodies were still being removed, and others being taken to local hospitals, he chose to be heartless and point the finger," Clay-Flores said.

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

It appeared the migrants had recently crossed the border and were picked up by the truck to be taken to where they would work, according to a Mexican official, the CBP official and another U.S. official, all of whom requested anonymity to discuss the matter.

Authorities were alerted to the truck through an emergency-911 call from a passerby flagged down for help by a migrant who had escaped from the truck, the officials said.

The victims were found sprinkled with a pungent substance, officials said, a practice some smugglers are known to use to mask the scent of human cargo and evade canine detection.

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

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.

Some survivors of human smuggling in the past have been taken into the custody of the U.S. Marshals Service to testify as witnesses.

IMMIGRATION CHALLENGES

The deaths highlighted 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.

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 a U.S. Immigration and Customs Enforcement investigative unit who retired in December.

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

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