Faulty Studs Led to Mexico City Metro Collapse, Attorney General Says

MEXICO CITY — Poorly placed studs, clear design flaws and deficient welding led to a collapse in the capital’s metro system last spring that left 26 people dead and scores injured, the attorney general for Mexico City said Thursday in releasing the results of a monthslong investigation.

The findings, detailed during a news conference, aligned with several of the conclusions of a New York Times investigation, which found that poor installation of the metal studs was central to the deadly May 3 accident.

The metal studs were key to the whole construction, creating a composite unit between steel beams and concrete slabs. Once the studs failed, the entire structure became fatally weakened, eventually leading to the collapse.

The studs “connecting the concrete slab with the metal beams were misplaced,” said Ulises Lara López, spokesman for the attorney general’s office. “In the sections where the concrete slabs were detached from the beams, all the studs were poorly installed at irregular patterns.”

Such fundamental flaws, Mr. Lara said, could not have been found through routine inspection or avoided through regular maintenance.

In releasing her office’s findings on Thursday, Attorney General Ernestina Godoy Ramos described the report as “a deep, serious, scientific investigation, carried out by experts.”

An independent report issued earlier this year by the Norwegian risk management company DNV also found serious construction errors, including with the placement and welding of metal studs. Mexico City’s government had hired DNV to examine the causes of the crash.

The crash of the metro has set off political shock waves, implicating some of the most powerful people in Mexico, including Foreign Minister Marcelo Ebrard, who was the mayor of Mexico City when the line was constructed.

Mr. Ebrard and the current mayor, Claudia Sheinbaum, are both expected to seek the presidency in the 2024 election.

The metro section that collapsed was built by Carso Group, a vast conglomerate owned by one of Mexico’s wealthiest and most powerful men, Carlos Slim, a close ally of President Andrés Manuel López Obrador. (Mr. Slim was also once a large shareholder of The New York Times Company.)

Ms. Godoy said her office would initiate criminal proceedings against people deemed responsible for the collapse, although she did not identify any individual. “Just as our mandate is justice, our priority has been and will continue to be the victims,” she said.