An explosion caused a factory building to collapse north of the Philippine capital, killing at least 12 workers and injuring three others, officials said Wednesday. Police, firefighters and rescue workers sifted through debris for one missing worker after the factory's steam boiler exploded late Tuesday, said Tony Mateo of the Municipal Disaster Coordinating Council in Bulacan province's Santa Maria township. The explosion ignited a brief fire at the factory, which produces Styrofoam packaging materials. The blast was so powerful that it destroyed half the building and sent debris flying outside the compound and onto the roof of a school across the street, Mateo said. Santa Maria police chief Marcos Rivero said five workers were immediately found dead at the site. Three more bodies were recovered overnight, and four of seven injured workers later died at the hospital, Mateo said.