MOSCOW — A suspected gas explosion at an apartment building in southern Russia killed at least one person and injured two others early Monday, authorities said. The blast on the top floor ripped through the nine-story building in the city of Shakhty, not far from the Ukrainian border, before dawn. One woman was killed and two people have been hospitalized, Rostov region governor Vasily Golubev said on state-owned Russian television. He said four people who are believed to live in the building are unaccounted-for, though it is unclear whether they were there at the time of the blast. Footage from the site showed that the top floors of the building had collapsed and damaged apartments below. The Investigative Committee said in a statement that it is treating the incident as a gas explosion and is searching the offices of the company that was the gas supplier for the building. The two injured people are in serious condition, Health Ministry spokesman Maxim Radetsky told the Rossiya 24 channel. One of the injured is a 2-year-old girl who was pulled out of the rubble.