A final report on the Mexico City metro collapse in May that killed 26 people confirmed that the devastating event was most likely caused by poorly welded and missing studs that connected steel support beams. 

The report by Norwegian certification firm DNV, which was commissioned by the Mexico City government to investigate the subway overpass collapse, found that poor installation of studs resulted in two independent parallel beams, a concrete beam and steal beam, that experienced loading conditions for which they were not designed, according to The Associated Press. 

“This created conditions that led to the distortion of the central transverse frame and the initiation and propagation of fatigue cracks that further reduced the capacity of the structure to support the load,” the report added, Reuters reported. 

The report confirmed similar conclusions made in a preliminary DNV report released in June, including that due to the overpass poor construction, it was already in a “compromised condition” before a major 2017 earthquake caused damage to parts of the infrastructure. 

Reuters reported that Jesus Esteva, head of Mexico City’s public works department, said that the government is already working on repairs to the subway line. 

“In the next few days we will be signing the agreements with the companies, he said. They will be doing the work. 

The $1.3 billion subway line was originally built between 2010 and 2012 through a partnership between Mexican construction company ICA,  French train producer Alstom SA and Grupo Carso, a company run by the family of Mexican business tycoon Carlos Slim.

During its construction, the line had several reported unanticipated costs and alleged design flaws, and the AP noted that Mexico City closed down the line in 2014 just 17 months after it opened due to needed replacements and repairs. 

Mexican President Andrés Manuel López Obrador had said in June that Carso would conduct repairs on the line without charging the government, adding that the line could be re-opened in the next year. 

Scrutiny and backlash over the subway lines construction ramped up following the May collapse, with many questions aimed at Foreign Affairs Secretary Marcelo Ebrard, who served as Mexico Citys mayor at the time of the subways construction. 

Allegations of poor design and construction first appeared shortly after Ebrard, who is currently considered a possible contender to succeed López Obrador, left the mayor’s office in 2012.