At least 53 mostly Central Americans were killed when the lorry they were in overturned in southern Mexico, in one of the worst accidents to befall migrants risking their lives to reach the United States.
The trailer broke open and spilled out migrants when the lorry crashed on a sharp bend outside the city of Tuxtla Gutierrez in the state of Chiapas, according to civil defence authorities.
The Attorney General's office put the death toll at 53.
"It took a bend, and because of the weight of us people inside, we all went with it," said a Guatemalan man sitting at the scene in footage broadcast on social media.
"The trailer couldn't handle the weight of people."
Several dozen people were injured and taken to hospitals in Chiapas, which borders Guatemala. More than 100 people were inside the trailer, authorities said.
Men, women and children were among the dead, the Chiapas state government said. President Andres Manuel Lopez Obrador expressed his sorrow at the "very painful" incident.
Migrants fleeing poverty and violence in Central America typically trek through Mexico to reach the US border, and sometimes cram into large trucks organised by smugglers in extremely dangerous conditions.
"This shows us that irregular migration is not the best way," Kevin Lopez, a spokesman for Guatemala's presidency, told Milenio television.
He did not know how many Guatemalan victims there were.
El Salvador's foreign minister, Alexandra Hill, said her government was checking whether Salvadorans had died.
Mexico offered lodging and humanitarian visas to the survivors, and Chiapas Governor Rutilio Escandon said those responsible for the accident would be held to account.
Officials in Mexico routinely come across migrants packed into trailers, including 600 people found hidden in the back of two trucks in eastern Mexico last month.
The journey north from Mexico's border with Guatemala is perilous and expensive, and many migrants fall prey to criminal gangs en route. In January, 19 people, mostly migrants, were massacred with suspected police involvement in northern Mexico.
Record numbers of people have been arrested on the US-Mexico border this year as migrants seek to capitalise on President Joe Biden's pledge to pursue more humane immigration policies than his hardline predecessor, Donald Trump.
Authorities in Chiapas have attempted to persuade migrants to not form caravans to walk thousands of miles to the US border, and have begun transporting people from the southern city of Tapachula to other regions of the country.
The Biden administration has also urged migrants not to leave their homelands for the United States, and this week saw the restart of a policy initiated under Mr Trump to send asylum seekers back to Mexico to await their court hearings.
Some critics argue that tougher policies push migrants into the hands of human smugglers, putting their lives at risk.
The authorities "generate smuggled migration that generates billions of dollars in profits", said migrant activist Ruben Figueroa.