MEXICO CITY—A fire struck Mexico City’s downtown subway headquarters early on Saturday, officials said, as smoke billowed out of the multi-story building roof, causing at least one death as well as light injuries. Six lines of the capital’s sprawling subway lines were knocked offline due to the fire, which was believed to be caused by an electricity failure. Mayor Claudia Sheinbaum told reporters near the building that the early morning fire was “practically extinguished” as teams of firefighters and water tanks remained at the scene. Sheinbaum said one police officer, who had likely slipped and fallen into the flames, was confirmed dead. She said 30 people had been transported to local hospitals due mostly to smoke inhalation. Officials said the fire began around 5 a.m. local time, and investigators from the city government as well as Mexico’s national electricity company, CFE, were evaluating damages as well as the exact cause …