Two men have been charged in connection with the deaths of 51 migrants who were found in sweltering conditions in a semitruck in San Antonio, according to criminal complaints filed in US District Court for the Western District of Texas.

Juan Claudio D'Luna-Mendez and Juan Francisco D'Luna-Bilbao have been charged with “possession of a weapon by an alien illegally in the United States,” according to the criminal complaints that were filed Monday, the same day the migrants were found. Authorities were able to locate the men after responding to the semi-truck incident, according to the affidavit.

“Officers researched the Texas registration plate on the semi-truck and found a residence in San Antonio, Texas as the truck's registered address with Texas motor vehicle records,” the affidavit said.

The San Antonio Police Department set up surveillance at the residence “and observed a Ford F-250 leave the residence with a single Hispanic male driving.” The man was identified as D'Luna-Bilbao, who had a firearm in the console, the affidavit said.

At the same home, authorities saw another truck whose driver -- a younger man -- was identified as D'Luna-Mendez.

Both men are Mexican nationals residing in the US illegally, the affidavits said. CNN has been unable to determine if either man has an attorney.

On Monday, authorities were alerted to the scene just before 6 p.m., when a worker in a nearby building heard a cry for help, said Police Chief Bill McManus. The worker found a trailer with doors partially opened and saw people deceased inside, he said.

The truck went Monday through a checkpoint north of Laredo, Texas, said US Rep. Henry Cuellar, who represents a district including Laredo and San Antonio, which are about 150 miles apart. Cuellar spoke Tuesday with Homeland Security Secretary Alejandro Mayorkas and has been in touch with US Border Patrol, he told CNN.

Felipe Betancourt Jr., a co-owner of Betancourt Trucking and Harvesting in Alamo, Texas, told CNN Tuesday the semi-truck used the same federal and state identifying numbers as one of his vehicles. He said images of the truck show it was displaying his company's federal Department of Transportation number and the Texas DOT identifying numbers from one of his trucks. This was first reported by the San Antonio Express-News.

Betancourt said the truck in San Antonio, which is the same color as his red Volvo semi, is not owned by his company. A search of the US DOT and Texas DOT numbers posted on the truck found in San Antonio showed they numbers have been registered to Betancourt's company.

“We are not linked at all to that truck," Betancourt said. “We don't know who owns that vehicle."

The death toll includes migrants from Mexico, Guatemala and Honduras, according to a federal law enforcement official, who spoke on condition of anonymity.

The medical examiner's office has identified potentially 34 of 51 victims, Precinct 1 Bexar County Commissioner Rebeca Clay-Flores said during a news conference Tuesday. The county medical examiner has also asked for assistance from medical examiner offices in neighboring counties due to the large number of victims.

Forty-eight people died on the scene, and two died at hospitals, the federal law enforcement official told CNN on Tuesday, noting the toll is preliminary.

Sixteen people -- 12 adults and four children -- were taken alive and conscious to medical facilities, San Antonio Fire Chief Charles Hood said at Monday night's news conference.

“This is the worst human-smuggling event in the United States. 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. Now they are organized and tied in with the cartels. So you have a criminal organization that has no regard for the safety of the migrants. They are treated like commodities rather than people,” he told CNN in a phone interview.

The discovery came as US federal authorities launched what they described as an “unprecedented” operation to disrupt human smuggling networks amid an influx of migrants at the US-Mexico border.

Three people have been detained and are in police custody, though their connection to the situation is unclear, Police Chief Bill McManus said at a news conference Monday night. It is unclear if the two men charged are a part of the three people detained.

Migrants were 'too weak' from the heat to help themselves

Patients were hot to the touch and suffering from heat stroke and exhaustion, Hood said. The refrigerator semi tractor-trailer had no visible working air conditioning unit, and there was no sign of water inside, he said. It's not clear how long people inside the truck had been dead, the official said.

High temperatures Monday in the San Antonio area ranged from the high 90s to low 100s, according to the National Weather Service.

“None of these people were able to extricate themselves out of the truck,” Hood said. “So they were still in there, awaiting help, when we arrived ... meaning just being too weak -- weakened state -- to actually get out and help themselves.”

Ashley C. Hoff, US Attorney for the Western District of Texas, said the migrants were “the apparent victims of human smugglers indifferent to the well-being of human life.”

She added: “We will continue to work with the Homeland Security Investigations and the local responders to identify and bring those who were responsible for this tragedy to justice.”

Those in the truck included at least 22 Mexicans and two Hondurans, the federal law enforcement official said. Seven Guatemalans were among the dead, and another Guatemalan was in a hospital in critical condition, that nation's foreign Minister told CNN.

President Joe Biden described the discovery as “horrifying and heartbreaking,” saying the deaths underscored the need to go after criminal trafficking rings.

“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.

Firefighters saw 'stacks of bodies'

“I am heartbroken by the tragic loss of life today and am praying for those still fighting for their lives,” Mayorkas said on social media. “Far too many lives have been lost as individuals -- including families, women, and children -- take this dangerous journey.”

The 60 firefighters that were on scene are being put through a critical incident stress debriefing, Hood said.

“We're not supposed to open up a truck and see stacks of bodies in there. None of us come to work imagining that,” the fire chief said.

One of the bodies was outside the trailer when firefighters arrived, Hood said.

Business owners in the area where the trailer was found told CNN they were in shock.

They were human beings, it was terrible," said Israel Martinez, 68, co-owner of USA Auto Parts. “We (migrants) come to this country for a better life and yesterday reminded many of us that sadly, some of us achieve it but many others don't do it.”

US officials are working to better handle the flow of migrants to the US-Mexico border, Mayorkas told CNN earlier this month. Their operation builds on previous initiatives to go after smugglers whom migrants often depend on as they make their way to the border. Homeland Security last spring also announced an effort to crack down on criminal smuggling organizations, alongside federal partners.

Migrants in recent years have faced other tragedies and challenges enduring dangerous heat and terrain while trying to cross the US-Mexico border.

Rescues across the US southern border have outpaced those of the last fiscal year. Since October, more than 14,000 searches and rescues have happened along the US southern border, according to US Customs and Border Protection -- including rescues from dangerous water crossings. That's up from 12,833 searches and rescues in fiscal year 2021, with more than three months left to go.

In 2017, 10 people died and dozens were injured from heat-related conditions after being discovered in a tractor-trailer at a San Antonio Walmart. The driver of the truck was sentenced to life without parole in a federal prison.

In 2003, 18 victims ranging from age 7 to 91 were found dead in the back of a semitruck 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 almost 34 years in prison.

Heat has not been the only danger to migrants crammed in vehicles. In March 2021, a semitruck packed with 25 people collided with an SUV on an isolated stretch of California's Imperial Valley, killing 13 undocumented immigrants.

•PHOTO: Police and other first responders work the scene where officials say dozens of people were found dead Monday in San Antonio.

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