MEXICO CITY, Mexico (AP) — An elevated section of the Mexico City metro collapsed and sent the train plunging toward a busy boulevard late Monday, killing at least 23 people and injuring at least 79, city officials said.

Rescuers found four bodies trapped 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 23. A crane was brought in yesterday to lower the car.

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

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 line in question was built, and the collapse could represent a major blow for him.

“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 that was trapped on the road below, adding that 77 of the injured were hospitalised. She also said children were among the dead.

The overpass was about five metres (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 on the road below.

The Mexico City Metro — which is among the world’s cheapest with tickets costing about 25 cents — has had at least two serious accidents since its inauguration half a century ago. In March of last year, a collision between two trains at the Tacubaya station left one passenger dead and injured 41 people. In 2015, a train that did not stop on time crashed into another at the Oceania station, injuring 12.

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

Adrián Loa Martínez, 46, said that 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 toward the site.

Gisela Rioja Castro, 43, was looking for her husband, 42-year-old Miguel Ángel Espinoza. She said that her husband always takes that train after finishing work at a store, but he never got home and had stopped answering his phone. When she heard what happened, she immediately feared the worst but has got no information from the authorities.

“Nobody knows anything,” she said.

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

Allegations about poor design and construction on the subway line emerged soon after Ebrard left office as mayor. Ebrard leads Mexico’s efforts to obtain coronavirus vaccines and has been considered a potential presidential candidate in 2024 elections. The line had to be partly closed in 2013 so tracks could be repaired.

Ebrard 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.”

Ebrard, yesterday, 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 yesterday and hundreds of buses were called in.