An explosion thought to have been caused by a gas cylinder or a heating boiler ripped through the state-run Marcelino Ugalde School in this mining center outside Bilbao, in northern Spain. At least 67 people, mostly children, were killed, and scores more were trapped under the collapsed two- story building. Officials say the number of dead and injured could reach 100. Many of the victims were trapped in a basement classroom by the blast. The school, which has three main teaching buildings, had 700 registered pupils. It was not immediately known how many were inside when the explosion occured. The explosion was apparently caused by a welding accident.