Mexico City (CNN)Construction flaws led to the collapse of a Mexico City subway line that killed 26 people and left dozens injured last month, according to the preliminary findings of an independent probe released by the city government on Wednesday.
The investigation, carried out by Norwegian risk management firm DNV, suggests that deficient welding of metal studs, which apparently were not well connected to steel beams supporting a concrete slab and the elevated train rails, was among a number of issues that contributed to the May 4 incident.
The report said missing metal studs in some sections of the structure, different kinds of concrete used for the slab and unfinished or badly welded joints were some of the other factors that caused the raised railway to buckle, sending two subway carriages plummeting to the streets below.
Jesús Esteva, Mexico City's public works and services secretary, said the accident was caused by a "structural failure" due to "deficiencies in the construction process."
Ceramic rings that fit around the studs were not removed before they were attached to the structure, he added.
Eckhard Hinrichsen, DNV's director general in Mexico, said this was the first report of three; subsequent reports will be released on July 14 and August 30.
The Mexico City Attorney's Office, as well as the College of Civil Engineers of Mexico, are conducting investigations into the incident.
Mayor Claudia Sheinbaum said her goal was to get Line 12, where the accident took place, operating again "as soon as possible under conditions of maximum safety."
She said her government had assembled a team of Mexican experts of "the highest level" to come up with "reinforcement and rehabilitation plan for Line 12" with the support of President Andrés Manuel López Obrador.
Sheinbaum said that victims are being taken care of and will receive, "full reparation for the harm [suffered]," with a city official assigned to each family so that "no one is left without support."
Mexico City's subway system is one of the busiest in the world. Line 12, also known as the "Golden Line," spans 25.1 kilometers (15.6 miles) and features 20 stations. It was touted as one of the most expensive and ambitious public works projects in Mexican history when it was inaugurated in October 2012.
But, in the years since, it has become a symbol of Mexico's social ills and challenges from corruption to impunity, inequality to negligence.
The Golden Line was supposed to connect historically marginalized neighborhoods with more prosperous areas of the Mexican metropolis and give people equal access to jobs, cultural centers and some of the best things Mexico City has to offer in an affordable way.
But several experts who spoke with CNN in the days following the collapse said it was a tragedy foretold. The dozens of deaths and injuries caused by the collapse were the culmination of a long series of red-flag incidents that should have prompted authorities to suspend service or shut down the line completely, commuters said.