Tuesday, Mexico's president, Andrés Manuel López Obrador, promised an in-depth investigation to find those responsible for the deaths of at least 25 people in the collapse of the elevated metro train line with a history of problems.

The promise came as emergency services in Mexico City worked to retrieve the bodies of victims still trapped under the wreckage of carriages that plunged to the ground Monday in a cloud of dust.

Dozens were injured in the incident, one of the worst ever to strike the capital's metro, raising questions about construction and maintenance standards on a network used by millions on a daily basis.

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

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

The metro line, Mexico City's newest, was built while Foreign Minister Marcelo Ebrard, a close ally of Lopez Obrador, was mayor of the capital between 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 handful of metro stations on the same line because repairs were required due to excessive wear.

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

The section where Monday's accident happened was built by a company belonging to the Mexican billionaire, Carlos Slim.