MEXICO CITY — The death toll from the collapse of an overpass on the Mexico City metro rose to 24 Tuesday as crews untangled train carriages from the steel and concrete wreckage that fell onto a roadway.

Another 77 people were injured when the support beams collapsed Monday night as a train passed along the elevated section, said Mayor Claudia Sheinbaum in describing one of the deadliest incidents in the history of the subway system, one of the world’s busiest.

On Tuesday, a crane lowered a train car containing four bodies to the ground.

Of the 24 killed, 21 died at the scene, while the others died at hospitals. Children were among the fatalities, Sheinbaum said.

Mexican Foreign Relations Secretary Marcelo Ebrard called the collapse “the most terrible accident we have ever had in mass transportation.” Ebrard was Mexico City’s mayor from 2006 to 2012, when the affected line was built.

“The cause of the collapse of the beam must be established through an expert investigation,” Sheinbaum said.

The overpass was about 16 feet above the road in the borough of Tlahuac, but the train ran above a concrete median strip, which apparently lessened the casualties among motorists.

The Mexico City Metro, which is among the world’s cheapest with tickets costing about 25 cents, has had at least three serious accidents since its inauguration a half-century ago.

In March 2020, a collision between two trains at the Tacubaya station left one passenger dead and injured 41. In 2015, a train that did not stop on time crashed into another at the Oceania station, injuring 12. In October 1975, at least 26 people were killed in a metro accident.

Dramatic video of Monday’s incident showed the overpass collapsing in a shower of sparks, sending up a cloud of debris as one of the train cars smashed into a vehicle on the road below.

Hundreds of police officers and firefighters cordoned off the scene as desperate friends and relatives of people believed to be on the train gathered. They crowded together waiting for news despite a coronavirus outbreak in Mexico City.

Many families were directed to the Belisario Dominguez Hospital, about a quarter of a mile away, where they waited for updates on the victims.

Floodlights illuminated the collapsed bridge as search and rescue teams tried to find survivors in the rubble. Ambulances, firefighters, the military and Mexico’s forensic department came and went from the scene while a helicopter hovered overhead as dawn broke.

“I’m looking for my son,” the mother of a 13-year-old boy told a local television station through tears. “He was on the subway — I can’t find him.”

Adrian Loa Martinez, 46, said his mother called him to tell him that his half brother and sister-in-law were driving when the overpass collapsed and that a beam fell onto their car.

He said that his sister-in-law was sent to a hospital, but that his half brother, Jose Juan Galindo, was crushed and he feared he was dead.

“He is down there now,” he told journalists, pointing to the site.

Gisela Rioja Castro, 43, was looking for her 42-year-old husband, Miguel Angel Espinoza.

She said he always takes that train after work, but he never got home and had stopped answering his phone.

When she heard what happened, she immediately feared the worst but has gotten no information from the authorities.

“Nobody knows anything,” she said.

The collapse occurred on Line 12, the subway’s newest. It stretches far into the city’s south side. Like many of the dozen subway lines, it runs underground through more central areas of the city of 9 million but is on elevated concrete structures on the outskirts.

Allegations of poor design and construction on the subway line emerged soon after the Ebrard left office as mayor. The line had to be partly closed in 2013 so tracks could be repaired.

Ebrard, who leads Mexico’s efforts to obtain coronavirus vaccines, has been considered a potential presidential candidate in 2024.

“What happened today on the Metro is a terrible tragedy,” he posted on Twitter.

“Of course, the causes should be investigated and those responsible should be identified,” he wrote. “I repeat that I am entirely at the disposition of authorities to contribute in whatever way is necessary.”

On Tuesday, Ebrard appeared at President Andres Manuel Lopez Obrador’s daily news conference, saying, “I share the indignation that exists.”

It was not clear whether a 7.1 magnitude earthquake in 2017 could have affected the subway line. There were reports that cracks had appeared in the base of at least one of the columns that support the elevated track following that earthquake.

The line was closed Tuesday and hundreds of buses were called in to carry thousands of commuters.

The New York Times contributed.