The collapse of a Mexico City metro overpass that left 26 people dead was caused by a structural failure, according to an initial investigation by independent experts published Wednesday. The report by Norwegian engineering company DNV, which was presented at a news conference by Mexico City authorities, identified a number of deficiencies in the construction process linked to the May 3 disaster. They include problems with the welding, insufficient bolts and the use of different types of concrete. The metro line involved in the tragedy, in which dozens were injured when a section of elevated track collapsed and a train came crashing down, is the city's newest. It has been plagued by problems since it was inaugurated in 2012 by then-mayor Marcelo Ebrard, who is now Mexico's foreign minister. The disaster has prompted accusations of negligence and demands for justice from devastated relatives. axm-dr/bfm