A full investigation will be carried out into the causes of an overpass collapse that killed at least 23 people when a Mexico City metro train plunged onto a busy road below, authorities said on Tuesday.

Two train carriages precariously hung from the damaged overpass after the accident late on Monday, and rescue efforts were briefly suspended with authorities worried that more train parts and debris could slam down onto the road.

Firemen using heavy chains to stabilize the site pulled bodies and survivors from the wreckage. The injured were taken to nearby hospitals in the south of the city.

A video on Milenio TV channel showed the overpass plummeting onto a stream of cars near the Olivos station in the southeast of the city at around 10.30 pm local time, sending up clouds of dust and sparks.

The crash, which left at least 60 people injured, has raised questions about safety on one of the world’s busiest metro systems, which spreads across a vast urban sprawl that is home to some 20 million people.

It was the second serious accident this year after a fire at a central control building knocked out service on several lines for weeks.

Speaking at a regular news conference, Mexican president Andres Manuel Lopez Obrador said that the investigation should be done quickly and nothing hidden from the Mexican people.

Mayor Claudia Sheinbaum, who earlier said it appeared a girder had given way on the overpass, said the attorney general’s office and an external company would both be involved in getting to the bottom of what happened.

Foreign Minister Marcelo Ebrard, who was city mayor when the overpass was built, said it was the most “terrible” accident to have hit the local transport system, and that he was ready to cooperate with authorities in the investigation.

At the conference with Lopez Obrador, Sheinbaum and Ebrard faced repeated questions from reporters about who should be held accountable for what happened.

Both urged the public to allow investigators to do their work before seeking to apportion responsibility for the accident.