Mexico’s president on Tuesday promised an in-depth investigation to find those responsible for the deaths of at least 23 people in the collapse of an elevated metro train line with a history of problems.

The vow came as emergency services in the capital worked to retrieve the bodies of victims still trapped in the wreckage of the carriages that plunged towards the ground on Monday night.

Dozens were injured in the accident, one of the worst ever to strike the Mexico City metro, raising questions about construction and maintenance standards on a network used by millions every day.

"A thorough investigation will be carried out ... to know the truth,” President Andres Manuel Lopez Obrador said at his daily news conference, adding that independent international experts would assist prosecutors in the probe.

"We cannot get into speculation, much less blame the possible perpetrators without having proof,” he added.

The metro line, the city’s newest, was built while Foreign Minister Marcelo Ebrard, a close ally of Lopez Obrador, was mayor of the capital from 2006-2012.

"For my part, I put myself at the full disposal of the authorities”, said Ebrard, considered among the possible ruling party candidates for the 2024 presidential election.

In 2014, Ebrard’s successor as mayor, Miguel Angel Mancera, suspended services at a dozen stations on the same metro line after wear was detected on the track and train wheels.

A study later concluded that there were problems with the design, operation and maintenance of the track.

‘Don’t know anything

Carriages were seen hanging from the metro overpass in the south of the capital in a tangle of twisted cables.

Several minors were among the 23 dead, Mexico City Mayor Claudia Sheinbaum told reporters.

Four of the victims were still in the train, she added.

Nearly 80 others received hospital treatment following the dramatic accident.

Earlier, anxious relatives had gathered at the site awaiting news of those believed to have been aboard the train.

One man said that his brother was among those trapped.

"He came with his wife and they managed to get her out, but he was crushed there and we don’t know anything,” he said.

Dozens of emergency workers were seen trying to rescue victims from the carriages.

The work had to be suspended because of fears the wreckage was too unstable, but it later resumed with the help of a crane.

A car was trapped under the rubble, but a person inside managed to get out alive, Sheinbaum said.

She promised a structural examination of the affected metro line, which will remain closed, and a full investigation into the causes of the accident.

"Citizens have the right to know the truth,” said Sheinbaum, who is also seen as a ruling party contender for the 2024 presidential vote.

Lucky escape 

One man, Jose Martinez, told reporters that he had a miraculous escape because he was unable to leave work in time to catch the ill-fated train.

"I was saved by like 15 minutes. It’s good that nothing happened to me,” he said.

Ricardo de la Torre, a Mexico City resident who lives close to Line 12, said that he had been worried about the quality of the overpass because the trains make nearby buildings shake.

"By that simple fact we know that the construction is bad,” he said.

Fernando Espino, leader of the subway workers union, told the Milenio television channel that engineers had already reported failures on the line on various occasions.

"It could have been negligence. They didn’t take it seriously,” he added, noting that unlike other metro lines number 12 is maintained by an external firm.

Monday’s incident comes just over a year after two subway trains collided in Mexico City, leaving one dead and around 40 injured as panicked passengers escaped through dense smoke.

In January of this year, one person died and 29 suffered smoke inhalation injuries in a fire in the metro’s control center.

In one of the worst ever accidents on the network, two metro trains rammed into each other leaving 23 dead and 55 injured in October 1975