Authorities say it may have begun when a drunk customer at the nightclub fired a flare gun on the dance floor, Russian media report.It took emergency workers several hours to put out the fire, which began at around 02:00 local time (23:00 GMT) in Kostroma, 300 km (180 miles) northeast of Moscow.Russia's investigative committee said a 23-year-old man had been arrested on suspicion of using a "pyrotechnic" object inside the building which led to the fire.Tass news agency quoted a law-enforcement source as saying that the suspected perpetrator had admitted responsibility.Images of the single-storey building engulfed in flames were shown on Russian state television, which said some safety measures had not been properly observed."