MEXICO CITY, Sept 7 (Reuters) - A deadly Mexico City metro collapse in May was caused in part by missing bolts in beams in an overpass that already had deficiencies before a major earthquake, according to an independent auditor"s report released on Tuesday by the city government. The 180-page analysis by Norwegian company DNV was the latest installment of its technical opinion on the May 3 collapse - Mexico"s biggest train accident in years - that killed 26 people when an overpass and train carriage on Metro Line 12 suddenly plummeted onto a stream of cars near the Olivos station in the southeast of the city. In the second phase of DNV"s findings shared by city officials, the firm said deficiencies including the lack of functional bolts over a significant stretch led to the buckling of north and south beams. The deficiencies left the structure operating as two independent girders that took on weight it was not designed to handle. "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 said. Sections of the collapsed overpass were in "compromised condition" before a major 2017 earthquake that caused damage to parts of the metro, the report said. Poor welding practices were also observed by the auditor. The city government has already started working on rehabilitation of the line, and the report"s findings will be shared with a technical advisory committee, said Jesus Esteva, head of Mexico City"s public works department. "In the next few days we will be signing the agreements with the companies. They will be doing the work," Esteva said, without adding further details. The metro was built by a consortium of Mexico"s ICA (ICA.MX), Grupo Carso, a company controlled by the family of Mexican tycoon Carlos Slim, and French trainmaker Alstom SA (ALSO.PA). Grupo Carso had no immediate comment on the report. A spokesperson for ICA did not immediately respond to a request for comment and a representative for Alstom could not immediately be reached The collapse put pressure on close allies of President Andres Manuel Lopez Obrador, as well as on Slim, Latin America"s richest man whose construction company was responsible for building the part of line that collapsed. Carso would repair the line at no cost to the government so that it can re-open in a year, Lopez Obrador said in June. read more DNV had been due to present its follow-up report on Aug. 23, but requested a two-week extension to complete its probe. DNV"s initial report found "six deficiencies in the construction process" that contributed to the accident and noted inadequate bolts and deformed structural supports. read more The company is still expected to deliver a third phase of its findings on the metro collapse.