SAN ANTONIO (NewsNation) — At least 51 people are dead, including 39 men and 12 women, after authorities discovered dozens of migrants abandoned in the back of a tractor-trailer near San Antonio, Texas on Monday.

Mexican Foreign Secretary Marcelo Ebrard C. tweeted an update Tuesday morning, saying 22 of the victims were Mexican, seven were Guatemalan and two were Hondurans. Others have yet to be identified.

Ebrard also said that Mexico has joined DHS with the investigation in the U.S.

Ebrard also confirmed President Andrés Manuel López Obrador will meet with President Joe Biden on July 12.

The bodies, believed to be those of migrants attempting to cross into the U.S., were found along with 16 survivors who were transported to area hospitals with apparent heat exhaustion.

“The patients that we saw were hot to the touch, they were suffering from heat stroke, heat exhaustion – no signs of water in the vehicle – it was a refrigerated tractor-trailer but there was no visible working AC unit on that rig,” San Antonio Fire Department Chief Charles Hood said.

According to officials, a city worker passing on a back road heard a cry for help just before 6 p.m. and alerted authorities. Once officers arrived on scene, they found saw a body outside of the truck, with the trailer’s gate partially opened.

First responders who then looked into the back of the tractor-trailer discovered “stacks of bodies,” Hood said.

The tractor-trailer was reportedly involved in a presumed migrant smuggling attempt. San Antonio Police Chief William McManus said it’s the largest incident in the city’s history.

Three people have been taken into custody in connection to the investigation, which has been handed over to federal officials with the Homeland Security Investigations, McManus said. Authorities are uncertain what role the trio played in the 46 deaths.

San Antonio Mayor Ron Nirenberg called the event a “horrific human tragedy,” saying he believes those who died were likely families trying to find a better life.

Calling the deaths a Nirenberg added, “More than 40 hopeful lives were lost. I urge you to think compassionately, pray for the deceased, the ailing, and their families at this moment,” Nirenberg said.

Dozens of first responders who were called to the scene are now receiving mental health services.

“We’re not supposed to open up a truck and see stacks of bodies in there. None of us come to work and imagine that,” Hood said.

The Associated Press reports that the incident may be the deadliest tragedy among thousands who have died while trying to cross the U.S. border from Mexico.

In 2017, 10 migrants died after becoming trapped inside a truck parked at a San Antonio Walmart store. In 2003, 19 migrants were located in a truck southeast of San Antonio.

Big rigs emerged as a popular smuggling method in the early 1990s amid a surge in U.S. border enforcement in San Diego and El Paso, Texas, which were then the busiest corridors for illegal crossings.

Heat poses a serious danger, particularly when temperatures can rise severely inside vehicles. Weather in the San Antonio area was mostly cloudy Monday, but temperatures approached 100 degrees.

The Associated Press contributed to this report.