Originally Published: 27 JUN 22 21:57 ET

Updated: 28 JUN 22 16:02 ET

By Amy Simonson, Amanda Musa, Travis Caldwell, Priscilla Alvarez and Amir Vera, CNN

(CNN) -- Fifty migrants are dead after they and others were found in sweltering conditions in a semitruck in San Antonio, a US Immigration and Customs Enforcement spokesperson said Tuesday.

The death toll, which the city's fire chief had given Monday as 46, includes migrants from Mexico, Guatemala and Honduras, according to a federal law enforcement official, who spoke on condition of anonymity.

"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 who 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 detained away from the trailer site are in police custody, though their connection to the situation is unclear, Police Chief Bill McManus said at a news conference Monday night.

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

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.

Patients were hot to the touch and suffering from heat stroke and exhaustion, Hood said. The refrigerator semitractor-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."

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.

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.

Mexico's attorney general will investigate the deaths, the country's foreign minister said on Twitter.

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.

Assistance to Mexican nationals would be provided, Mexico's consul general in San Antonio said on social media. At least two of the 16 survivors have been identified as Guatemalan, according to Mexico's foreign minister, citing the consul.

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.