TUXTLA GUTIÉRREZ, Mexico — The Guatemalan teenager had been packed with more than 150 fellow migrants for hours, he said, jammed in rows of six, some sitting, some standing, some choking on the southern Mexico heat.
Then the speeding tractor-trailer started to fishtail uncontrollably, said the teenager, Esvin Chipel Tzoy. Within seconds the vehicle flipped and crashed, the deadliest single-day disaster in many years to befall Central American migrants who attempt the perilous route through Mexico to the United States.
Mexico officials said at least 55 people had been killed and 106 hospitalized in the Thursday crash. They attributed the disaster to excessive speed and said the driver, who may have passed undetected through immigration checkpoints, escaped after the crash.
Interviews on Friday with survivors, witnesses and one of the first medics who rushed to the crash depicted a scene of mangled metal, vomit, puddles of blood and dust coating the bodies of migrants piled atop one another in the highway and what remained of the tractor-trailer.
Mr. Chipel said the tractor-trailer began to lurch from side to side and then he heard a loud boom, as if the brakes had failed, followed by the screech of metal as the trailer tipped. Then came the screams from fellow passengers, including children.
Not far behind, Melody Ramírez Moreno, 17, was perched behind her husband on a motorcycle when they saw the tractor-trailer sway precariously. Her husband hit the brakes but the bike’s front wheel started to twist, she said, as her foot became trapped and mangled in the back wheel.
“The only thing I could hear were the screams, the laments, the cries of the people on the truck,” she said. “Everything happened in the blink of an eye.”
The trailer overturned, slammed into a pedestrian bridge, split apart and scattered a mass of bodies across the highway, including Mr. Chipel himself.
“I couldn’t breathe,” Mr. Chipel said, recalling how his nostrils filled with blood and dust. “I thought I was dying.”
Mr. Chipel was among the luckier ones, with only a broken wrist and some cuts and scrapes.
The injured migrants, mostly from Guatemala like Mr. Chipel, were being treated at hospitals around Tuxtla Gutiérrez, the state capital, on Friday.
Mr. Chipel said he had left his hometown in Guatemala on Sunday and had been trying to get to the United States in hopes of getting work and supporting his aging parents, who disapproved of his decision to leave.
“In Guatemala, you can’t get ahead,” he said. “I wanted to go for that American dream.”
A paramedic who was among the first to arrive at the accident, Luis Eduardo Hernández Trejo, 21, said he could immediately tell many victims were lifeless, especially those still trapped inside the wreckage.
“They were all cadavers,” he said.
Realizing the severity, Mr. Hernández said he called for backup and sought to identify the wounded who needed the most urgent help.
The total number of occupants in the tractor-trailer remained unclear. Some survivors, bleeding and limping, fled the scene to avoid possible arrest by the immigration police in Chiapas, which borders Guatemala.
The accident revealed in graphic clarity the increasingly perilous journey that people from across Latin America endure to reach the United States border, risking ruthless criminal cartels, corrupt police officers and hostile terrain for the chance at a better life.
After helping with the triage, Mr. Hernández said he was called over by an immigration official to treat a migrant who had tried to flee into a nearby house but had begun feeling ill.
Two other young migrants had tried to flee the scene toward a nearby river, one of them with a cut on his head.
“He was scared,” Mr. Hernández said. “He thought we were with the government.”
He also recalled seeing a young mother with her infant son sitting by the roadside, blood streaming from a head wound. The baby, who survived, was the only infant on the truck, according to Mr. Hernández.
Nearby residents offered support, he said, bringing supplies like cotton balls and alcohol to help the paramedics treat the wounded.
“It was a tragic incident,” Mr. Hernández said. “The people were trying to get to the United States for a better life.”
Heavily armed National Guard troops surrounded the crash site, the twisted debris from the upturned truck and the bodies of victims long removed. The only telltale signs of the previous day were red streaks of dried blood on the road and a makeshift memorial of candles and fruit.
“It’s a feeling of deep sadness just to think of how many families were destroyed, torn apart,” said Giovanni Lepri, representative of the United Nations refugee agency in Mexico. At the same time, he said, “it’s horrifying to think that 54 people or more had to unfortunately die to give this much visibility to something that is happening every day.”
The crash caused the worst known single-day death toll for migrants in Mexico since the 2010 massacre of 72 migrants by the Zetas drug cartel in the northern state of Tamaulipas.
The severity may have political implications, underscoring the desperate and dangerous measures migrants are taking, even as governments in both Mexico and the United States attempt to stop the flow northward.
The U.S. government has in recent years implemented a series of policies taking a harsher approach to deterring migrants. Officials in the Biden administration have repeatedly and explicitly told them not to come.
“I am saddened to see the tragic loss of life and injuries of migrants traveling in Chiapas,” said U.S. ambassador Ken Salazar on Twitter on Thursday night. “Human smugglers disregard human life for their own profit. Please don’t risk your lives to migrate irregularly.”
Under former President Donald J. Trump, Washington began immediately returning many detained migrants under an obscure health code known as Title 42, while also forcing asylum seekers to wait in Mexico for their cases to be decided in U.S. courts, a program known as Remain in Mexico. Both policies have been continued under the Biden administration.
The U.S. government also has increasingly relied on Mexico to detain and deport migrants. As of October, the Mexican authorities had detained more than 220,000 undocumented migrants this year, the highest number on record.
The efforts from both sides of the border have had some impact: apprehensions at Mexico’s northern border, which reached a record high earlier this year, have begun tapering off.
However, as governments ramp up deterrence efforts, migrants have sought out increasingly dangerous means to evade the checkpoints set up by Mexican officials, paying smugglers steep prices to be crammed inside trucks and trailers and gain a greater possibility of avoiding detection.
This year has already been the deadliest for migrants at the U.S. border since 2014, according to the International Organization for Migration, which said this week that 650 people had died trying to cross so far in 2021 — the highest toll since the I.O.M. began keeping records.