Manila - Twelve factory workers were killed and five injured when a boiler at a Styrofoam packaging plant exploded in the Philippines, police said Wednesday.

   The explosion occurred Tuesday evening as night shift employees worked at the factory in Santa Maria town in Bulacan province, just north of Manila.

   The town's police chief, Superintendent Marcos Rivero, said five workers died on the spot while others were trapped under concrete and steel debris as the blast destroyed the factory.

   "The explosion was very powerful," he said. "Some of the fatalities were buried in rubble while others expired while undergoing treatment in nearby hospitals."

   Rivero said rescuers were still checking if there were still workers trapped in the debris.

   He said an initial investigation showed the boiler overheated, causing it to explode. Investigators have so far not found evidence of sabotage.