Before the line opened, it faced technical and structural issues that persisted during its operation, resulting in a partial closure of the elevated section where the accident occurred, lasting from 2014 to 2015.

Claudia Sheinbaum, the head of government of the city at the time of the collapse, hired the Norwegian risk management firm Det Norske Veritas (DNV) to investigate the causes of the event.

Preliminary findings linked the accident to bridge construction deficiencies, including a lack of functional studs and faulty welds, fatiguing the collapsed beam.

[7][6][8] In December 2018, the recently appointed head of government of Mexico City, Claudia Sheinbaum, designated Florencia Serranía [es] as the general director of the STC.

A congressional report indicated that coupling unprotected rails with substandard wheels could lead to train derailments,[7][22] and it received its safety certification one hour before its operation started.

[19] In March 2014, the city government closed the elevated section for twenty months for repairs due to technical and structural faults,[29] and established a special board to investigate the errors that led to it.

[32][33] In a non-public report published by SinEmbargo.mx after the collapse, SYSTRA had noted that the wear and tear was unusual for a project that was only two years old, which they attributed to "mediocre quality" work.

The group concluded that the Metro system would need to address the root causes of the wear to avoid future problems, as repairs would only serve as temporary fixes.

They notified government authorities that a permanent resolution to the issues was infeasible and recommended developing a maintenance manual customized to the line's requirements for continuous operation.

[35] According to Serranía, the French company Tricaud Société Organisée (TSO) was contracted in 2016 to maintain the daily operating conditions of the line's fixed installations and reported no concerns before the collapse.

[41] By the end of that year, the engineering firm Ingeniería, Servicios y Sistemas Aplicados (ISSA) conducted a study on the structural and geotechnical behavior of the overpass, concluding that there was no risk to the line's operation.

[42][43] Approximately 220 meters (720 ft) before reaching Olivos station,[44] the section of track between columns 12 and 13 collapsed due to a failure of the girder, causing the rear two train cars to derail.

To enhance the service, the city contracted additional buses from Autobuses de Oriente, and added temporary bus routes to Tasqueña, Universidad, Atlalilco, and Coyuya metro stations.

[21][77] In the days following the collapse, protesters expressed anger over the incident and the authorities' responses by vandalizing several Metro stations, breaking glass partitions and painting slogans such as "It wasn't an accident—It was negligence" on the walls.

[81] On the eve of the first anniversary of the collapse, the National Action Party (PAN) installed a temporary sculpture on Paseo de la Reforma in front of the Antimonumento +43 memorial.

[83] The office of the federal Attorney General (FGR), along with its Mexico City counterpart (FGJCDMX) led by Ernestina Godoy Ramos, and the Norwegian risk management firm Det Norske Veritas (DNV) were appointed to investigate the collapse.

[86][87] According to the contract, DNV was authorized to "collect information for documentary analyses, covering design, construction, rehabilitation, intervention, operation, maintenance, and supervision" of the line up to the date of the collapse.

[90] It found an alert issued by supervisors in August 2010 regarding the construction of the Tezonco–Olivos overpass, which said that only ten reinforcement bars were to be used in the bridge, despite the original project plans specifying twenty.

[86][93] In the 180-page investigation, DNV concluded that the collapse was caused by a lack of functional Nelson studs in the affected section, which resulted in the buckling of the north and south beams.

[97][98][99] Andrés Lajous, the city's Secretary of Mobility, explained that the change in the report's methodology pertained to DNV's decision to "plant maintenance" as a cause of the collapse.

The firm added that Héctor Salomón Galindo Alvarado, the referred lawyer, was hired in July 2019 and had no authority over the reports or investigations conducted by the company or being contracted by DNV México.

[101][102] On 9 May 2022, the Spanish newspaper El País published part of the third report, in which DNV concluded that the root cause of the accident was the bridge's design not complying with construction standards.

[106] Three days before DNV released its report, journalists from The New York Times (NYT) published an investigation attributing the collapse to a series of errors originating in its planning.

It also noted that the city approved substandard work and that audits after the 2017 earthquake revealed errors in the original construction, including missing steel components and improperly poured concrete.

[112] They noted a lack of inspection reports addressing bridge issues, stating that existing deformities and defects could have been identified with proper maintenance and specialized examinations.

[107][21] The study found that 68 percent of the elevated section exhibited minor and common deficiencies, while the remaining 32 percent—between Periférico Oriente and Zapotitlán stations—showed vulnerabilities requiring further analysis.

[120][121] A week later, López Obrador announced that Carso had agreed to rebuild the collapsed section at no cost,[122][123] and Slim remarked: I am convinced that [the line] was built by the best engineers in Mexico, who did the calculations and the design, and if you remember, in October 2012, around November ... the project was approved, which had been done by international experts.

[135] Just days before the second report's delivery, DNV filed a complaint with the FGJCDMX due to a breach in the chain of custody regarding a package sent to the United States for further studies.

[138][139] Horcasitas' defense argued that errors arose when the city sought to modify the original project and contracted different companies, claiming that the line's infrastructure had not received proper maintenance since its opening.

[144] By February 2022, 80 percent of those affected had received compensation from Carso as part of a deal stipulating that they would not pursue legal action or seek additional payments.