MEXICO CITY – A Mexico City metro accident that killed 26 people in May was caused by flaws in the rail line’s construction, according to a city-commissioned report, opening billionaire Carlos Slim’s infrastructure firm to scrutiny.

The tragic late-night collapse that plunged a train from an elevated track into traffic below was due to poor or unfinished welding and the mixing of different types of concrete in the structure, Norwegian firm DNV said in its report. The structure was missing various bolts compared with the original design.

The report’s focus on construction failures throws Slim’s Grupo Carso SAB into the spotlight because it built the portion of the Line 12 metro that collapsed, raising questions about the future of the company’s dealings with the government. A unit of Grupo Carso is also building part of President Andres Manuel Lopez Obrador’s flagship Maya Train project through the country’s southeastern region.

Family spokesman and Slim’s son-in-law Arturo Elias Ayub declined to comment. Carso’s stock fell 1% to 65.29 pesos on Wednesday.

The law requires “restitution, rehabilitation, compensation,” for the victims’ families, Mexico City Mayor Claudia Sheinbaum said at a press briefing where she presented the report.

Both Foreign Affairs Minister Marcelo Ebrard, who was mayor at the time of the metro line’s inauguration in 2012, and Sheinbaum, have come under scrutiny amid media reports that the construction was rushed or maintenance had failed.

DNV said that the line was operating under normal conditions before the accident and the rails appeared to be adequately maintained.

Both Ebrard and Sheinbaum are seen as early contenders for the 2024 presidential elections.

The New York Times published similar findings on Sunday about faulty construction, including that ceramic rings used around studs during installation in the concrete slabs had not been removed.

In his morning briefing on Wednesday, the president criticized the story by The New York Times, which said the metro’s construction may have been rushed because Ebrard sought to open the metro before his mayoral term ended in 2012. Lopez Obrador said the report “lacked ethics.”

“There will be punishment for those responsible, whoever is responsible” for the accident, Lopez Obrador said.

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