Hundreds of emergency workers combed the site of a five-story apartment building in southern Ukraine Thursday after a series of explosions reduced it to rubble, authority said. The blasts Wednesday night in a Black Sea resort town of Yevpatoria left at least 17 people dead and 24 others missing, according to Igor Krol, a spokesman for the Ukraine’s Emergency Situations Ministry. 21 people have been rescued. “We are now investigating all possible reasons for the explosions,” Krol said. Volodymiyr Shandra, the Ukrainian Emergency Situations minister, told local media that oxygen canister being stored in the basement of the building could have triggered the blasts. Television footage showed the rescuers trying to free people buried underneath fallen debris, while others scrabbled through wires, construction rods and boulders.