TIM REID AND ALEXANDRA ULMER A deadly rampage by a 67-year-old suspect in Northern California was likely an act of workplace violence, authorities said, offering fresh details about the second in back-to-back mass shootings that killed 18 people and horrified the nation. The two shootings within three days have left the state of California reeling as investigators worked to identify the 11 people killed on Saturday at a Los Angeles-area dance hall and the seven killed on Monday in the seaside town of Half Moon Bay near San Francisco. Authorities were trying to determine the motives for both shootings. Officials said preliminary evidence indicated…