MEXICO CITY — On a balmy night in May, Tania Lezama Salgado hopped on the metro with her sister Nancy after spending hours looking for the grandest pink dress and the sparkliest shoes possible for her 15th birthday party.

Tania had grown accustomed to the screeches and shakes of the metro, but as it barreled across an overpass that night — jerking violently, going faster than she had ever remembered — something felt different.

A New York Times investigation shows the serious construction flaws and political pressure behind a tragedy in Mexico City.

A New York Times investigation shows the serious construction flaws and political pressure behind a tragedy in Mexico City.

Suddenly, she heard a loud bang, then screams, as the overpass collapsed and the train plummeted about 40 feet to the street below. When Tania came to, her neck was wedged between the doors of the metro, her head poking out of the wreckage, the smell of blood curling into her nostrils.

Bodies strewn on top of her, her outstretched hands felt what seemed to be the straps of her sister’s backpack. As she pulled, she said, she discovered they were the entrails of another passenger.

Tania now spends her days in the hospital, unable to walk, her shattered pelvis held together by a metal contraption, four screws poking out of each side of her body. Above her hospital bed is a photo of her 22-year-old sister Nancy — one of 26 people who died in the metro crash that night.

Soon after, President Andrés Manuel López Obrador of Mexico, who positions himself as a champion of the poor and an enemy of the elite, apologized to the victims’ families and urged patience while officials examined what went wrong and who was to blame.

“The humble, hardworking, good people understand that, unfortunately, these things happen,” he said during a news conference Tuesday.

But a New York Times investigation — based on years of government records, interviews with people who worked on the construction, and expert analysis of evidence from the crash site — has found serious flaws in the basic construction of the metro that appear to have led directly to its collapse.

The disaster has already spiraled into a political crisis, threatening to ensnare two of the nation’s most powerful figures: the president’s foreign secretary, Marcelo Ebrard, and one of the world’s wealthiest businessmen, Carlos Slim.

Ebrard was mayor of Mexico City when the new metro line, known as the “Golden Line,” was built, a heralded expansion of the second-largest subway in the Americas that could boost his credentials for a possible presidential run. And Slim’s construction company, Carso Infrastructure and Construction, built the portion of the line that collapsed — the firm’s first rail project, paving the way for more.

The Times took thousands of photographs of the crash site and shared the evidence with several leading engineers who reached the same conclusion: The steel studs that were vital to the strength of the overpass — linchpins of the entire structure — appear to have failed because of bad welds, critical mistakes that likely caused the crash.

That is one of the primary explanations being considered by Mexico City officials, according to several people familiar with the official investigations into the disaster, and it underscores a pattern of political expediency and haphazard work as the metro was being built.

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Now Slim’s company is building a significant part of another signature project: Tren Maya, a 950-mile railway championed by López Obrador that is supposed to revitalize southern Mexico and help cement his legacy as president. But some engineers working on it say they are encountering problems that are similar to the ones they faced when they built the subway that collapsed.

The Times reviewed thousands of pages of internal government and corporate documents on the metro’s troubled history, finding more than a decade of warnings and concerns about safety before the fatal crash.

  • In a rush to finish, the city demanded that construction companies open the subway well before Ebrard’s term as mayor ended in 2012. The scramble led to a frenzied construction process that began before a master plan had been finalized and produced a metro line with defects from the start. The outcry over the problems was so intense that Ebrard eventually moved out of the country for 14 months, leaving behind what he called a “political witch hunt.”
  • Federal auditors found that the city “authorized poor quality work” even as the line was being built. The metro was certified less than an hour before it was inaugurated, even though thousands of pieces of work had not been completed, according to a 2014 investigation by the city’s Legislative Assembly.
  • During an inspection after a major earthquake in 2017, the city found errors in the original construction of the section built by Slim’s company, including incorrectly poured concrete and missing steel components, according to an unreleased government document from 2017.
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First responders on the tracks on the morning after the collapse.

The night of the crash, Tania said the metro shook so much, “it was like it was dancing.”

That same phrase had been used seven years ago — when city officials grew so concerned about the new subway, known as Line 12, that they shut down part of it in 2014 — only about 17 months after it was inaugurated.

“The line was dancing,” said Joel Ortega, the metro director at the time. The train, he said, “was floating over the tracks.”

When Ebrard announced the new metro on an August day in 2007, he crowned it the “Golden Line,” promising to build the most modern stretch of subway in Latin America, with access for disabled riders, internet, day care centers — even a museum.

Ebrard vowed that it would change the lives of poor and marginalized people, offering the young mayor a chance to leave his stamp on the Mexican capital.

“It was a great social project,” said Moisés Poblanno, who worked for Ebrard at the time and remains a close aide. “Marcelo did a lot of things in government, but the most important, by far, was Line 12.”

His ambitions ran parallel to those of another giant in Mexican society: Slim, the magnate who sought to expand his empire into the lucrative rail industry. Line 12 was his company’s initial venture into the sector.

“I’m always trying to build up experience on our resume,” Antonio Gómez García, CEO of Group Carso, Slim’s sprawling conglomerate, said in an interview. “Participating in the construction of the line, or in any bid of this kind and winning it, allows you to be a part of the next one.”

But major problems with the subway emerged early on. Pushing to finish before Ebrard left office, the city bought train cars that arrived quickly but were not compatible with the rail line; the wheels did not fit properly on the track. The result was a constant pounding, warping and rippling of what should have been smooth steel track.

Less than a year after certifying the line as safe, auditors produced a report documenting a litany of problems: cracked and broken parts, deformed tracks and a relentless pummeling of the tracks.

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The damage had become so severe that it “put users’ safety at risk,” Ortega elaborated in testimony after shutting down the line. After more than a year of repairs, the city reopened it in 2015. But problems persisted, with concerns intensifying after a horrific earthquake in 2017.

Some feared the flaws were never resolved, hidden too deep within the structure.

“This line was born with cancer,” said Jorge Gaviño, the metro director during the earthquake in 2017. “The definition of hidden defects is that you can’t see them; on the surface, everything looks good, but there are problems underneath.”

In a statement to the Times, Ebrard said that “the issues observed” during construction of the metro line did not affect its operation. He suggested the cause of the crash may have been tied to maintenance, saying it was impossible to know whether his successor “conducted all of the maintenance work required in the event of earthquakes of a certain magnitude.”

“Line 12, which for years has benefited millions of people, is perhaps the most audited public work in the history of Mexico,” Ebrard said.

But evidence from the crash site indicates that the metro’s flaws ran much deeper than maintenance.

Underneath the tracks, the line that carried more than 250,000 people around the Mexican capital every day was held together by boltlike studs. Welded into steel and encased in concrete, they created a structure much stronger than either material on its own.

The strength of the overpass depended on those studs; they were an essential connection keeping it intact.

But photographs of the rubble point to a fundamental lapse during construction: The welds holding everything together were far too weak. Photographs show that the studs broke clean off the steel beams, creating what engineers called an unstable structure incapable of supporting the train.

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“A good quality weld would not have failed like that,” said Gary Klein, a member of the National Academy of Engineering and an executive at Wiss, Janney, Elstner, a firm that studies construction-related failures.

The studs were handled shoddily, with little attention to detail, engineers reviewing the photographs said. One obvious mistake: Workers never removed many of the ceramic rings used around the studs during installation. Many of those rings can be seen embedded in the concrete slabs that collapsed that night.

Beyond that, the placement of the studs was so irregular that it looked more like a crew’s improvisation than a strict adherence to a construction plan, according to the engineers who reviewed the photos.

When the metro took its last journey, the entire overpass became fatally compromised.

“When the studs failed, the concrete and the steel did not work together anymore,” said Donald Dusenberry, a consulting engineer who has investigated many bridge collapses. “The steel became overwhelmed.”

After an earthquake devastated the capital in 2017, the city did its own inspection and found other construction flaws in the portion built by Slim’s company. An internal government document from 2017 reviewed by the Times noted that parts of the overpass had “structural faults” and were missing steel components and that some of the concrete was badly poured, an error that suggested “carelessness.”

Gómez García, CEO of Group Carso, Slim’s sprawling empire, acknowledged in an interview that leaving the ceramic rings around the studs was not ideal but said it had not affected the structure.

“There’s something called hidden flaws; in the end, these are things that can happen,” Gómez García said. “Unfortunately, there they are in the photos. They didn’t remove them.”

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Gómez García said he believed the studs sheared off only after the overpass crashed to the ground and “were not the cause of the accident.” He added that maintenance could be to blame, as the equipment and materials used to repair the line after it was shut down in 2014 were heavy, possibly putting too much stress on the overpass.

But several independent experts rejected that explanation, noting that the photographic evidence points directly to the weak welds as the likely cause of the accident.

Whatever the official investigations determine, the crash could carry immense consequences — not just for Ebrard and Slim (who was previously a large shareholder of The New York Times Co.). If maintenance played a role, voters may also blame the president’s protégé, Claudia Sheinbaum, who has overseen the metro as Mexico City’s mayor for the last two years.

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Claudia Sheinbaum, the mayor of Mexico City, last year.

As two of the most dominant figures in the governing party, Sheinbaum and Ebrard are widely expected to vie for López Obrador’s blessing to succeed him and run for president in the 2024 elections.

With the public outcry over the crash, Sheinbaum’s administration offered about $32,000 to each of the families that lost a loved one. But some have refused the money and are suing the Mexico City metro system instead.

“The line wasn’t made right,” said Bernarda Salgado López, the mother of Tania, who is still in the hospital, and Nancy, who died. Salgado has joined the lawsuit.

“I think they should be held responsible for what happened, for everything, for everyone who died,” she said.

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Bernarda Salgado López and Humberto Lezama, the parents of Tania, who is still in the hospital, and Nancy, who died.

From the beginning, it was clear that the government wanted to keep costs low on the metro and move as quickly as possible.

Originally, the idea was to build an entirely underground line, but it soon became evident that would be too costly and take too long. The city cut the budget for the metro line almost immediately.

A consulting firm hired by the metro system listed the pros and cons of an elevated line. It would be cheaper and faster but less safe. The city ultimately changed course and decided to go with a partially elevated line.

To win the contract in 2008, Slim’s company, Carso Infrastructure and Construction, joined two established firms: one of Mexico’s largest construction companies, ICA, and Alstom of France. Carso had no experience building train lines, but it did have two attractive qualities: liquidity in the midst of the global financial crisis and access to a steel business owned by Slim.

“It wasn’t just the money; it was the steel,” Gómez García said.

After construction began, federal auditors discovered serious flaws. In a 2009 report, they documented “badly executed work” performed “without quality controls,” among other issues, noting that “there was inadequate communication” between the groups supervising the project and the construction companies. Ebrard told the Times that “all observations” from the federal auditors “were resolved.”

The city put pressure on contractors to complete the job as quickly as possible. The companies faced a fine of about $120 million if they did not finish well before Ebrard’s term ended, according to Enrique Horcasitas, the project director. Construction began even before a master plan had been completed.

Ebrard noted that using “as-built” plans, in which companies draw up blueprints as they build, was common and “allowed for technical flexibility” without “compromising the integrity or safety of the project.”

But many of the engineers who worked on Line 12 and spoke with the Times said that most of the large-scale projects they had worked on, whether in Mexico or abroad, relied on master plans from the start.

“It wasn’t built with a master plan,” said Mario Alberto Ruiz, who worked as an engineer for ICA building the line, “and that was also the source of many failures.”

“We had to build as soon as possible above all because, as you well know, there was going to be a change of government,” he said.

The problem that drew the most public attention was the purchase of trains, which did not fit tightly enough on the tracks.

The mismatch stemmed from another timesaving decision. The rails were designed for American standards, according to testimony in the 2014 investigation by the city Legislature. But the government ended up choosing a Spanish supplier, CAF, that provided trains designed for European specifications. The reason: CAF had promised to deliver the trains about a year ahead of its competitor, Canada-based Bombardier.

“Bombardier gave us a longer timeline,” said Francisco Bojorquez, metro director at the time, in 2014 testimony. “It was a question of time and of opportunity.”

The incompatibility caused so much wear that the city had to replace a half-mile of rail weeks before the metro even started carrying passengers.

As the inauguration in 2012 drew near, engineers and their colleagues worked through the night, scrambling to get the line in presentable shape.

“There was a lot of pressure,” said Marcos Tapia Manjarrez, who also worked for ICA on the line. “We worked in shifts — I’m talking about almost 18 hours straight.”

The tension rose when Ebrard made visits to assess how much headway had been made, engineers said.

“They would say, ‘The mayor is coming,’ and obviously that put time pressure on us, because we hadn’t made enough progress,” Ruiz said.

Ruiz recounted installing provisional lighting just so there would be some way to light up the stations for the ribbon-cutting ceremony. The lighting was just for show; they disconnected it right after.

The city received the line’s safety certification less than an hour before Ebrard cut the ribbon, according to the final report by the Legislative Assembly in 2014.

“Today we have the honor of giving this public work to the people of Mexico City,” Ebrard said at the ceremony. “A public work that is complete, finished, with the most advanced technology in the world.”

A little over a year later, the line was partially shut down.

The metro director at the time, Ortega, said during the 2014 investigation that after just a year in service, parts of the metro line showed signs of wear that should be expected after a decade of use. The same company that certified the line conducted an inspection a year after and found more than a dozen problems, including “abnormal warping” of tracks and poor welding.

“In an accident,” Ortega said, “not only would the train run off the track, but we would probably have an enormous tragedy.”

On the night of the crash, as Tania and her sister returned from shopping, the train was crammed with passengers. Tania said she struggled to hold onto a pole as the train screeched across the track before plunging onto the road below.

Tania struggled to find her sister as injured passengers cried out for help. Only at the hospital did she learn of her sister’s death.

“I thought it was my fault,” she said, solemnly. “I had made us go look for my dress.”

As news of the collapse spread, a group of engineers working on Tren Maya, the new 950-mile railway being championed by the president — a significant stretch of which is being built by Slim’s company — took to a WhatsApp group chat. For months, the engineers had been trading concerns and frustrations, complaining about a disorganized and rushed construction process.

López Obrador inaugurated the construction last June, and the government tourism arm overseeing Tren Maya said in a statement that the project is being built with the “highest quality and safety standards,” although it acknowledged it was still making changes to the master plan.

“The Tren Maya project is a campaign promise of the current administration, and we have the commitment to finish it before the end” of López Obrador’s term as president, the agency said.

In hundreds of messages viewed by the Times, engineers have discussed construction progressing on Tren Maya without plans or details being approved, as well as unfinished designs.

And May 4, their attention turned to the crash on Line 12, which some of them had helped build. The engineers traded different theories about what had happened, but many seemed to agree on one point: The line never felt right.

“What a tragedy!” one engineer said, “Forewarned is forearmed … ”

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Wilted floral tributes at the entrance of the Olivos metro station near where the overpass collapsed.

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