An explosion rocked a Southern California office building today, killing one person and injuring three others, authorities said. Orange County Fire Authority Chief Brian Fennessy said 70 firefighters were dispatched in response to a 1:09 pm (local time) call from Aliso Viejo today. The blast involved a building under renovation but the cause was not immediately known, Fennessey said. The explosion blew siding off the walls, exposing insulation and framing. Hazardous materials investigators were on the scene. "The corner of that building, the whole bottom floor is pretty much blown out," fire Captain Tony Bommarito said. There was no immediate information about the person who was killed. The injured were taken to a hospital. Fennessey said children at a daycare across the street were not injured. Little children holding hands and others being pushed in rolling cribs were led by firefighters to a Target parking lot to be united with parents.