MEXICO CITY, June 16 (Reuters) - Preliminary findings of an independent investigation into the deadly collapse of a Mexico City metro rail line last month showed the accident was caused by a structural fault, a senior city official said on Wednesday. Jesus Esteva, head of Mexico City"s public works department, said the probe by Norwegian firm DNV observed deficiencies in building materials used including bolts and deformation of structural supports in the part of the line that collapsed. The initial report by DNV, an external auditor, found "six deficiencies in the construction process" that helped to bring about the accident. "The aforementioned allows us to submit, on a preliminary basis, that the incident was sparked by a structural fault," Esteva told a news conference. Officials did not permit questions from reporters. Further reports on the accident are due in the coming months, Mexico City Mayor Claudia Sheinbaum said. Deficiencies identified included the welding of bolts and their attachment to girders, missing bolts on some girders, different types of concrete used and unfinished or poorly executed welding, the city said. The collapse, Mexico"s biggest train accident in years, killed 26 people and put pressure on close allies of President Andres Manuel Lopez Obrador, as well as Mexican billionaire Carlos Slim, Latin America"s richest man. The incident put a spotlight on Slim"s construction business, Grupo Carso (GCARSOA1.MX), which was involved in building the section of the Line 12 metro route that collapsed. read more Grupo Carso said in a statement that it would not comment until the final report into the accident is published. Sheinbaum has faced questions about the line"s maintenance, and criticism has also been directed at Foreign Minister Marcelo Ebrard, who was the driving force behind the construction of Line 12 when he was mayor of the capital from 2006 to 2012. Both are close allies of Lopez Obrador and seen as possible successors to him in 2024 when his term ends. read more Responding to the initial findings, Ebrard published a two-page statement defending how his administration built the line and said that aside from the final technical report, a broader inquiry should be held to review the "entire decision-making process in the design, layout, supervision and maintenance of the work," as well as various studies on it in recent years. The metro was built by a consortium of Mexico"s ICA (ICA.MX), Grupo Carso and French trainmaker Alstom SA (ALSO.PA). Alstom last month said its role was limited to power supply, signaling, monitoring and control systems and some depot equipment, as well as testing some systems. The company will review the preliminary findings and had no further comment, a spokesperson said. ICA did not immediately respond to a request for comment. In May, ICA shared a document with a graphic showing it had built sections of Line 12 spanning 15 stations, and Carso five. Carso"s section included where the track fell. Sheinbaum said DNV will oversee analysis of the principal fault, and that she would be in touch with the companies involved in the line later today. Line 12 had repeatedly been reviewed and declared safe by officials since it opened in 2012, but problems led to partial shutdowns from 2014 to 2015 for repairs. Reporting by Dave Graham, Writing by Daina Beth Solomon