By Angelica Casas in San Antonio, Sam Cabral & Bernd Debusmann Jr.
"Poverty and desperation" led to the deaths of at least 50 migrants abandoned in a Texas lorry, Mexico's president has said.
Andres Manuel Lopez Obrador blamed trafficking and "a lack of control" at the border - the worst case of migrant deaths due to smuggling in the US.
Nearly two dozen Mexicans, seven Guatemalans and two Hondurans were among the dead.
Those found alive, including four children, were taken to hospital.
The survivors were "hot to the touch" and suffering from heat stroke and heat exhaustion.
So far, Mexican authorities have said that at least two Mexican citizens are being treated for dehydration in hospital. Consular officials are working to confirm their identities.
Authorities are working to confirm the nationalities of remaining victims.
According to US authorities, three people "believed to be part of the smuggling conspiracy" have been taken into custody.
Speaking at his daily briefing, Mr Lopez Obrador, popularly known as Amlo, called the discovery a "tremendous tragedy", and said Mexico would work to repatriate the remains of its citizens.
This and other migrant deaths were due to the "poverty and desperation of our Central American brothers, and of Mexicans," he said, adding: "It happens because there is trafficking of people and a lack of control, in this case at the Mexican-US border, but also in the US interior".
Mexico's Foreign Minister, Marcelo Ebrard, said that Mexico has joined the probe and is sending a team to Texas to help with the investigation.
The White House called the truck deaths "horrific and heart-breaking".
The migrants were found on the outskirts of the city of San Antonio, which is 250km (155 miles) from the US-Mexico border, is a major transit route for people smugglers.
Human traffickers often use trucks to transport undocumented migrants after meeting them in remote areas once they have managed to cross into the United States.
"They had families... and were likely trying to find a better life," San Antonio Mayor Ron Nirenberg said. "It's nothing short of a horrific, human tragedy."
Officials said a city worker had discovered the scene after hearing cries for help.
Emergency responders initially arrived at the scene at about 18:00 local time (23:00 GMT), San Antonio fire chief Charles Hood told reporters.
"We're not supposed to open up a truck and see stacks of bodies in there. None of us come to work imagining that," he said.
He added that the vehicle, which had been abandoned by its driver, had no working air conditioning and there was no drinking water inside it.
San Antonio's climate is blisteringly hot in the summer months, with temperatures there reaching 39.4C (103F) on Monday, and it is suspected that the victims likely died from heat exhaustion and dehydration.
The country's consulate in San Antonio has said it will provide "all the support" needed, dispatching Consul General Rubén Minutti to the scene.
Edward Reyna, a security guard at a lumber yard just metres away, said he was not surprised to arrive for his night shift and hear the news. He said he had lost count of the times he had seen migrants jumping off the train that passes right next to where the truck was found.
"I thought sooner or later, somebody was going to get hurt," Mr Reyna said. "The cartels that bring them over don't care about them."
This story has played out in Texas many times, but not to this magnitude. In 2017, 10 immigrants were found dead inside a similar tractor trailer outside a San Antonio Walmart - also on the city's south side. In 2003, 17 were found dead in a hot car in Victoria, another south Texas town.
San Antonio's far south side is a corridor with two main roads connecting the city to Texas border towns.
Mostly rural communities, a few junkyards and a handful of developing neighbourhoods in this part of San Antonio make it easy for a truck this size to go unnoticed - until it doesn't.
US Secretary of Homeland Security Alejandro Mayorkas, whose department has taken over the investigation, said: "Human smugglers are callous individuals who have no regard for the vulnerable people they exploit and endanger in order to make a profit."
Immigration is a contentious political issue in the US, where in May a record 239,000 undocumented migrants were detained crossing into the country from Mexico - many travelling along extremely risky and unsafe routes.
At least 650 migrants crossing the border died last year, according to the United Nations' International Organisation for Migration.
US law enforcement officials are on track to exceed the record 1.73 million border arrests made in 2021, with large numbers of people continuing to cross from Central American countries such as Honduras, Guatemala and El Salvador.
Fleeing poverty and violence in Central America, many of the undocumented migrants end up paying huge sums of cash to people smugglers to get them across the US border.
Over recent years, there have been many similar examples of migrants perishing during their journey, but no single event as deadly as what was discovered on Monday.
