Four men have been charged in connection with the alleged migrant smuggling operation that took the lives of 53 people who were trapped in the sweltering heat of a tractor-trailer in Texas.
Homero Zamorano Jr., 45, of Pasadena, Texas, was charged with one count of alien smuggling resulting in death on Wednesday. Zamorano was allegedly the driver of the truck that was found outside San Antonio. Mexican investigators said the driver allegedly tried to pass himself off to authorities as one of the surviving migrants.
On Tuesday, police arrested Christian Martinez, 28, in Palestine, Texas, after they discovered he was in contact with Zamorano about the alleged smuggling operation.
If convicted, Zamorano and Martinez face up to life in prison and possibly the death penalty.
Martinez had a court appearance on Tuesday and is being transported to San Antonio, while Zamorano has a scheduled court appearance for Thursday, authorities said.
Two other men have been arrested in connection with the truck deaths on gun charges, according to federal authorities.
Juan Claudio D'Luna-Mendez and Juan Francisco D'Luna-Bilbao were identified as unauthorized migrants in possession of multiple weapons, according to federal authorities.
D'Luna-Bilbao was traced to the semi-truck when he was seen near the residence connected to the truck's registration, according to a U.S. Bureau of Alcohol, Tobacco, Firearms and Explosives affidavit. After he was stopped by police, Bilboa allegedly admitted to possessing a firearm, according to court documents.
D'Luna-Mendez was also stopped near the residence connected to the semi-truck's registration, where he allegedly admitted to possessing multiple firearms at the home.
D'Luna-Mendez and D'Luna-Bilbao have detention hearings scheduled for Friday. They face a maximum sentence of 10 years in prison plus fines on the ATF charges.
The incident unfolded in the southcentral Texas city on Monday evening at around 5:50 p.m. local time, when a nearby worker heard a cry for help and found the tractor-trailer with the doors partially opened and the bodies of 46 people inside, according to San Antonio Police Chief Bill McManus and San Antonio Fire Department Chief Charles Hood.
The trailer was refrigerated but did not have a visibly working air-conditioning unit and there were no signs of water inside, according to Hood.
An additional 16 people -- 12 adults and four children -- had been transported to area hospitals in what officials called a "mass casualty event."
The victims taken to hospitals were hot to the touch and all suffering from heat stroke and heat exhaustion, Hood said. There were no child fatalities that authorities know of so far, he added.
"They suffered, horrendously, could have been for hours," Hood said.
Chris Magnus, the commissioner of U.S. Customs and Border Protection, told reporters he was "horrified" by the incident.
"Horrified at this tragic loss of life near San Antonio," Magnus said Monday. "This speaks to the desperation of migrants who would put their lives in the hands of callous human smugglers who show no regard for human life."
Of the 53 bodies in the custody of the medical examiner's office, 40 are male and 13 are female, the Bexar County Medical Examiner's Office said Wednesday
Rebeca Clay-Flores, the Bexar County Precinct 1 commissioner, said at a press conference Tuesday that some of those found are under the age of 18, likely teenagers.
Thirty-seven of the victims have potential identification, officials said.
U.S. Immigration and Customs Enforcement said those who have been identified so far were from Mexico, Guatemala and Honduras. The criminal investigation remains ongoing, as Homeland Security Investigations and its partners continue to work to identify all of the victims, according to ICE.
It's the deadliest incident of human smuggling in U.S. history, an HSI spokesperson told ABC News on Tuesday.
Mexican Foreign Secretary Marcelo Ebrard, citing information provided by U.S. authorities, said the dead included 22 Mexican citizens, seven Guatemalan citizens and two Honduran citizens. The other victims have yet to be identified and Mexico is working with the U.S. on an investigation, according to Ebrard.
"We are in mourning," Ebrard said in a statement Tuesday via Twitter. "Huge tragedy."
Hood told ABC News that the the smell of meat tenderizer, which was reportedly put on top of the bodies before the suspects fled, was overwhelming.
Hood said there were personal items near where the bodies were found, including prayer cards in Spanish and a new pair of Air Jordans.
President Joe Biden issued a statement Tuesday calling the deaths "horrifying and heartbreaking," blaming the criminal smuggling industry for preying on migrants. Biden also highlighted the anti-smuggling campaign the U.S. has launched with its partners, saying they have made more than 2,400 arrests.
"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.
Texas Gov. Greg Abbott confirmed Wednesday that the truck had not been inspected by Border Patrol, despite passing through a border checkpoint.
"It was not inspected because the Border Patrol does not have the resources to be able to inspect all of the trucks," Abbott said.
Abbott announced that the Texas Department of Public Safety will add additional truck checkpoints, beginning immediately. He said they will target trucks like the one involved in the migrants' deaths.
Bexar County Sheriff Javier Salazar sent Biden a letter, requesting a meeting and assistance. Salazar wrote he was "angry" that he has made several appeals to the administration, without response. He also criticized the "lack of action" that has allowed Abbott to use this as a "campaign stunt."
Making statements Wednesday, Abbott blamed Biden, saying Biden was warned in advance that reduced border enforcement would lead to dire consequences. Abbott said those consequences are a record number of people crossing the border illegally, a greater sense of lawlessness coming from not enforcing the law, increased brazenness by cartels because the federal government is not pushing back against them and the death of the 53 people on the truck.
"Many of these deaths could be prevented if Biden simply fully funded the border patrol operation of the United States of America and implemented the policies that the border patrol needs in order to do their real job and their real job is not the paper-processing work that they have been assigned to do. Their real job is both to secure the border as well as to do things like inspect the vehicle that was carrying those people who lost their lives," Abbott said.
ICE said initially that HSI agents found more than 40 deceased individuals upon arrival at the scene on Monday when responding to a call from the San Antonio Police Department regarding "an alleged human smuggling event."
"HSI continues its enforcement efforts to ensure the safety and well-being of our communities," ICE said in its statement. "We will continue to address the serious public safety threat posed by human smuggling organizations and their reckless disregard for the health and safety of those smuggled. To report suspicious activity, we encourage people to call the HSI Tip Line at 1-866-DHS-2ICE. All calls are kept confidential."
HSI is the arm of ICE responsible for taking down smuggling networks.
The San Antonio Fire Department confirmed to ABC News that HSI and CBP are taking over the investigation from local authorities.
CBP is the umbrella agency of the U.S. Border Patrol, which responded to assist at the scene and is supporting ICE in the federal investigation, according to Magnus, the CBP commissioner.
"We will be working with our federal, state and local partners to assist in every way possible with this investigation," Magnus told reporters Monday night.
White House press secretary Karine Jean-Pierre said the Biden administration will "continue to take action to disrupt human smuggling networks which have no regard for lives."
"Our prayers are with those who tragically lost their lives, their loved ones, as well as those still fighting for their lives. We are also grateful for the swift work of federal, state and local first responders," Jean-Pierre told reporters Tuesday.
When asked about the criticism from Republicans, including Abbott, who say Biden's border policies have led to dangerous journeys for immigrants, Jean-Pierre said the White House is focused on the victims and their families.
"But the fact of the matter is, the border is closed, which is in part why you see people trying to make this dangerous journey using smuggling networks," Jean-Pierre said.
Homeland Security Secretary Alejandro Mayorkas took to Twitter to say that he was "heartbroken by the tragic loss of life today and am praying for those still fighting for their lives."
"Far too many lives have been lost as individuals -- including families, women, and children -- take this dangerous journey," he tweeted Monday night. "Human smugglers are callous individuals who have no regard for the vulnerable people they exploit and endanger in order to make a profit. We will work alongside our partners to hold those responsible for this tragedy accountable and continue to take action to disrupt smuggling networks."
Earlier this month, the Department of Homeland Security released more details on the Biden administration's efforts to combat human smuggling and unauthorized migration in conjunction with the Summit of the Americas held in Los Angeles.
The series of operations launched across the Western Hemisphere is part of the largest human smuggling crackdown ever seen in the region, with more than 1,300 deployed personnel and nearly 2,000 smugglers arrested in just two months.
Agencies from across the administration, including the intelligence community and the U.S. Treasury Department, have engaged to disrupt smuggling operations in real-time and strip down the financial backing of the transnational criminal organizations that coordinate these crimes.
"The Biden administration is focused on putting these organizations out of business," DHS said in a recent statement prior to Monday's incident. "But human smuggling is, by definition, a transnational problem and we are committed to working with our regional partners in the Americas to commit our collective expertise and resources to put an end to human smuggling."
ABC News' Luke Barr, Marilyn Heck, Matt Gutman, Robert Zepeda, Anne Laurent, Scottye Kennedy and Josh Margolin contributed to this report.