Rescuers found four bodies inside a rail car that was dangling from the overpass but were unable to remove them. It was not clear if those dead were included among the toll of 23. A crane carefully lowered the car to the ground Tuesday.

Mayor Claudia Sheinbaum said one of the concrete beams collapsed as a train passed over it about 10:30 p.m. in one of the deadliest episodes in the history of the city’s subway system, which is among the world’s busiest.

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.

Earlier, she said someone had been pulled alive from a car on the road. She said 77 of the injured were hospitalized, adding that children were among the dead.

The overpass was about 5 meters (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 half a 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.

Hundreds of police officers and firefighters cordoned off the scene as desperate friends and relatives of people believed to be on the train gathered. Despite the fact that the coronavirus situation remains serious in Mexico City, they crowded together waiting for news.

Adrián Loa Martínez, 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 José 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 Ángel 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.

He wrote on Twitter: “What happened today on the Metro is a terrible tragedy.”

“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 Andrés Manuel López 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 quake.

The line was closed Tuesday and hundreds of buses were called in. Thousands in surrounding neighborhoods lined up before dawn to catch the buses for work.