TUXTLA GUTIERREZ, Mexico (AP) — Rescue workers arriving at a road accident in southern Mexico found a horrific scene: A tractor trailer jammed with as many as 200 migrants crashed into the base of a steel pedestrian bridge, killing 55 and injuring dozens.

The migrants inside the tipped-over trailer were tossed and crushed in a pile of both the living and the dead.

In addition to the 55 killed Thursday evening, at least 52 were injured. It was one of the deadliest days for migrants in Mexico since the 2010 massacre of 72 people by the Zetas drug cartel in the northern state of Tamaulipas.

Volunteer rescuers removed the dead from the pile, while the living scrambled to get out of the twisted debris of the collapsed trailer.

One young man, pinned beneath unmoving bodies, wriggled to free the lower half of his frame, his face wrenched into a grimace as he extracted himself. Nearby, a man blinked, unable to move as he lay on the side of the road. Next to him was an older, stouter migrant whose lifeless eyes stared into the setting sun.

While the Mexican government is trying to appease the United States by stopping caravans of walking migrants and allowing the reinstatement of the "Remain in Mexico" policy, it has been unable to staunch the flood of migrants stuffed by the hundreds into trucks operated by smugglers who charge thousands of dollars to take them to the U.S. border — trips that all too often lead them only to their deaths.

Mexican President Andrés Manuel López Obrador said such cases are painful.

"We have been insisting that the causes that originate these unfortunate events must be addressed," he said at his morning news conference.

López Obrador said he has been insisting with U.S. President Joe Biden that the immigration situation is urgent, but that the U.S. government is moving slowly.

The most severely injured from the accident were carried to plastic sheets on the road. Those who could walk were led, stunned, to the same sheets. Ambulances, cars and pickup trucks were pressed into ferrying the injured to hospitals.

Later, the dead were covered in white sheets, side by side, on the highway.

Rescue workers who first arrived said other migrants who had been on the truck when it crashed had fled for fear of being detained by immigration agents. One paramedic said some of those who hurried into surrounding neighborhoods were bloodied or bruised but still limped away in their desperation to escape.

About 200 migrants may have been packed into the truck, said Guatemala's top human rights official, Jordán Rodas. That number is not unusual for migrant smuggling operations in Mexico, and the weight of the load — combined with speed and a nearby curve — may have been enough to throw the truck off balance, authorities said.

Luis Manuel Moreno, head of the Chiapas state civil defense office, said about 21 were seriously injured and were taken to hospitals. The federal Attorney General's Office said three were critically injured in the crash, which happened on a highway leading from the Guatemalan border toward the Chiapas state capital.

Sitting beside the overturned trailer, Celso Pacheco of Guatemala said the truck felt like it was speeding and then seemed out of control.

Most aboard were from Guatemala and Honduras, he said, estimating eight to 10 young children among them. He said he was trying to reach the United States, but now expected to be deported to Guatemala. Authorities said there also were migrants from Ecuador, the Dominican Republic and Mexico aboard.

Marco Antonio Sánchez, director of the Chiapas Firefighter Institute, said ambulances brought victims to three hospitals, three or four at a time. When there weren't enough ambulances, they loaded them into pickup trucks, he said.

Guatemalan President Alejandro Giammattei tweeted: "I deeply regret the tragedy in Chiapas state, and I express my solidarity for the victims' families, to whom we will offer all the necessary consular assistance, including repatriation."

Pope Francis, who visited Chiapas in 2015 and has made the plight of migrants a hallmark of his papacy, sent a telegram of condolences Friday to the archbishop of Tuxtla Gutierrez, offering prayers for the dead and their families, and for the injured. 

The truck had originally been a closed freight module of the kind used to carry perishable goods. The container was smashed open by the impact. It was unclear if the driver survived.

Those who spoke to survivors said they told of boarding the truck in Mexico, near the border with Guatemala, and of paying between $2,500 and $3,500 to be taken to Mexico's central state of Puebla. Once there, they would presumably have contracted with another set of smugglers to take them to the U.S. border.

In recent months, Mexican authorities have tried to block migrants from walking in large groups toward the U.S. border, but the clandestine and illicit flow has continued.

In October, in one of the largest busts in recent memory, authorities in the northern border state of Tamaulipas found 652 mainly Central American migrants jammed into a convoy of six cargo trucks heading toward the U.S. border.

Irineo Mujica, an activist who is leading about 400 migrants on a nearly 1 1/2-month march across southern Mexico, blamed Thursday's tragedy on Mexico cracking down on migrant caravans.

Mujica and his group had almost reached the outskirts of Mexico City, after weeks of dealing with National Guard officers who tried to block the march. Mujica said the group would stop and offer prayers for the dead migrants.

"These policies that kill us, that murder us, is what leads to this type of tragedy," Mujica said.

In fact, they are two very different groups. Caravans generally attract migrants who don't have the thousands of dollars needed to pay smugglers.

Migrants involved in serious accidents are often allowed to stay in Mexico at least temporarily because they are considered witnesses to and victims of a crime, and Mexico's National Immigration Institute said it would offer humanitarian visas to the survivors.

The agency also said the Mexican government would help identify the dead and cover funeral costs or repatriation of the remains.

Mass deaths of migrants are something that President Andrés Manuel López Obrador has been desperate to avoid, even as his administration has accepted requests from the U.S. government to stem the flow of migrants moving north.

"It is very painful," he tweeted about the crash.