San Antonio CNN  — 

A distant cry led a worker Monday evening to a tractor-trailer abandoned on a desolate country road under the blazing Texas sun on the outskirts of the city.

On this barren stretch of scrubland adjacent to railroad tracks, the perilous journey north for dozens of undocumented migrants – many of them Mexican – ended in the back of a scorching tractor-trailer, nearly 150 miles north of the US border with Mexico.

In all, 53 people died in what one Homeland Security Investigations’ agent called the deadliest human smuggling incident in US history. Some victims could be younger than 18.

“This is nothing short of a horrific human tragedy,” said San Antonio Mayor Ron Nirenberg.

A local businessman described the back road where the semitruck was abandoned as “la boca del lobo” in Spanish, or “the mouth of the wolf,” because it is remote and pitch black.

The road runs parallel to Interstate 35, a major north-south route in the central United States for traffic and commerce from the southern border. The interstate stretches from Laredo, Texas, to Duluth, Minnesota, near the Canadian border. From San Antonio, it meanders north to Austin, Waco, Fort Worth and Dallas.

Law enforcement personnel investigate the tractor-trailer Monday in San Antonio.

It’s a route often exploited by smugglers at a time when record numbers of migrants are being intercepted at the US-Mexico border.

“This sheds light on how dangerous human smuggling is,” said Craig Larrabee, Homeland Security Investigations San Antonio acting special agent in charge.

“In the past, smuggling organizations were mom-and-pop,” Larrabee told CNN. “Now, they are organized and tied in with the cartels. So you have a criminal organization who has no regard for the safety of the migrants. They are treated like commodities rather than people.”

A cry for help leads to ‘stacks of bodies’

Just before 6 p.m. on Monday, a worker in a nearby building heard a cry for help and alerted local authorities to the abandoned truck, according to San Antonio Police Chief Bill McManus.

The doors to the hulking trailer were partially open when the worker arrived. Inside, he saw the bodies, the chief said.

Forty-eight people were dead at the scene and two died later at hospitals, said a federal law enforcement official, who spoke on condition of anonymity.

By Wednesday morning, the Bexar County Medical Examiner’s Office (BCMEO) said it had received a total of 53 bodies.

They were migrants from Mexico, Guatemala, Honduras and possibly El Salvador.

One body was found outside the trailer.

Inside the truck there were at least 22 Mexicans and two Hondurans, the federal law enforcement official said.

Seven Guatemalans were among the dead, and another Guatemalan was in critical condition at a hospital, according to the nation’s foreign minister.

“We’re not supposed to open up a truck and see stacks of bodies in there,” San Antonio Fire Chief Charles Hood said told reporters on Monday. “None of us come to work imagining that.”

Survivors, suffering from heat stroke, were hot to the touch

These dangerous and sometimes fatal human smuggling operations, transporting people in crammed trailers and vans with no air conditioning, are common along the southern border.

In 2017, 10 migrants died and dozens were injured from heat-related conditions in a tractor-trailer discovered at a San Antonio Walmart about three miles northeast of the latest incident. The driver of the truck was sentenced to life without parole in a federal prison.

Police and other first responders work at the place where dozens of migrants were found dead Monday in San Antonio.

On Tuesday, San Antonio resident Angelita Olvera left two colorful crosses in honor of the victims near the site of the latest tragedy.

“I didn’t know them,” she told CNN of the victims. “They are sons, mothers, fathers and grandchildren.”

Temperatures in San Antonio on Monday ranged from the high 90s to the low 100s.

Sixteen survivors – 12 adults and four children – were rushed to local hospitals. Suffering from heat stroke and exhaustion, the patients were hot to the touch, according to Hood. On Wednesday, 11 people were still in local hospitals.

The trailer had no air conditioning. There was no sign of water inside. It was unclear how long the victims had been dead.

“They were still in there, awaiting help, when we arrived … meaning just being too weak – weakened state – to actually get out and help themselves,” Hood said of the survivors.

Felipe Betancourt Jr., co-owner of a trucking company in Alamo, Texas, told CNN the semitruck abandoned on Monday used the same federal and state identifying numbers as one of his vehicles. The truck in San Antonio is the same color as his red Volvo semi, but is not owned by his firm.

A man pays his respects at a makeshift memorial for the victims in San Antonio.

Refrigerated semitrucks are insulated and meant to keep the temperatures stable, Betancourt said, but “if it’s carrying something hot inside, it won’t let the heat escape. The temperatures can reach up to 125-130 degrees when the doors are shut.”

On Monday, the truck went through a checkpoint north of Laredo, Texas, according to US Rep. Henry Cuellar, a Democrat whose sprawling district includes Laredo and San Antonio.

Homeland security officials are investigating the deaths, along with local police.

Three people were taken into police custody away from the trailer, Chief McManus said. They are believed to be part of the smuggling conspiracy, according to ICE.

Two men, Juan Claudio D’Luna-Mendez and Juan Francisco D’Luna-Bilbao, were charged federally with “possession of a weapon by an alien illegally in the United States” in connection with the incident, according to criminal complaints filed Monday in US District Court for the Western District of Texas. It is unclear if the two men charged are among the three people detained earlier.

Investigators at the scene traced the Texas registration plate on the semitruck and to a residence in San Antonio, the affidavit said. The suspects were arrested during traffic stops after leaving the residence, according to the complaints, and numerous weapons were recovered in a car and truck driven by the suspects.

‘Brothers and sisters who died following their hope of a better life’

The victims were 40 males and 13 females.

Some of them could be under 18 years old, according to Tom Peine, a Bexar County spokesman.

So far, the medical examiner’s office has potentially identified 37 people, BCMEO said in a statement Wednesday. Medical examiners in neighboring counties were assisting due to the number of victims.

Some victims had one photo ID while others had multiple IDs or none, complicating the identification process, Peine said.

Consular officials from Mexico, Guatemala and Honduras also vowed to help identify victims and assist survivors. The medical examiner was also in touch with the consulate of El Salvador though the nationalities of the victims are still being determined, Peine said.

“Far too many lives have been lost as individuals – including families, women, and children – take this dangerous journey,” Homeland Security Secretary Alejandro Mayorkas said on social media.

The Biden administration earlier this month launched what Mayorkas called an “unprecedented” operation to disrupt human smuggling networks amid soaring numbers of migrants at the southern border.

President Joe Biden described Monday’s discovery as “horrifying and heartbreaking.”

“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,” Biden said.

Pope Francis, via Twitter, urged prayers “for these brothers and sisters who died following their hope of a better life.”

650 died trying to cross US-Mexico border last year

Migrant rescues are increasing across the nation’s southern border.

Since October, more than 14,000 searches and rescues have occurred along the border with Mexico, according to US Customs and Border Protection – including those from dangerous water crossings. That’s up from 12,833 searches and rescues in fiscal year 2021, with more than three months left in this fiscal year.

At least 650 people died while trying to cross the US-Mexico border last year, the highest number since 2014, according to the International Organization for Migration, a United Nations agency.

Monday’s tragedy brings the total number of deaths for the first six months of the year to 290.

On Tuesday, helicopters buzzed over the desolate stretch of road where the trailer was abandoned as authorities searched for other migrants who might have been on the truck.

Olvera, the resident who left crosses near the scene, recalled joining neighbors in 2017 to pray for the 10 migrants who died in a broiling tractor-trailer parked at a Walmart.

She used to live in Piedras Negras, Mexico, Olvera said, fighting back tears, and is too familiar with the poverty some migrants have died fleeing.

It’s a tragedy that has repeated itself throughout the years. In 2003, 18 migrants, ranging in age from 7 to 91, were found dead in the back of a semitruck in Texas with about 100 other people as temperatures soared past 100 degrees, investigators said. The driver in that case was initially sentenced to life in prison, but in 2011 was resentenced to nearly 34 years in prison.

CNN’s Rosa Flores, Rosalina Nieves, Amir Vera, Joe Sutton, Amanda Musa, Travis Caldwell, Carolyn Sung, Michelle Watson, Karol Suarez, Kevin Liptak, Jason Hanna, Sharif Paget, Jen Deaton, Amanda Jackson and Steve Almasy contributed to this report.