Mexican President Andres Manuel Lopez Obrador on May 4 promised a full investigation involving international experts into a metro accident that left at least 23 dead in the capital.

"A thorough investigation will be carried out … to know the truth," Lopez Obrador said at his daily news conference.

"From that the responsibility will be established," he added.

The left-wing populist said that the investigation was in the hands of the federal and Mexico City prosecutors.

"We cannot get into speculation, much less blame the possible perpetrators without having proof. We will have the investigation carried out soon," Lopez Obrador said.

The metro line that partially collapsed on May 3 night was built while Secretary of External Affairs Marcelo Ebrard, a close ally of Lopez Obrador, was mayor of the capital from 2006-2012.

Ebrard told the same news conference that it was "a very sad day for everyone" and said he shared the outrage and pain over the tragedy.

"For my part, I put myself at the full disposal of the authorities," he said.

"I understand that there are many motivations of a political nature," added Ebrard, considered among the possible ruling party candidates for the 2024 presidential election.