HALF MOON BAY, (Calif): A deadly rampage by a 67-year-old suspect in Northern California was likely an act of workplace violence, authorities said on Tuesday, 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 motives for both shootings. Officials said preliminary evidence indicated that the massacre in Half Moon Bay appeared to be a case of workplace violence, and that the gunman had used a legally-owned weapon.

The suspects in both attacks are men of retirement age, much older than typical perpetrators of deadly mass shootings that have become routine in the United States.

On Monday, a gunman in Half Moon Bay opened fire on employees at Mountain Mushroom Farm, where he had previously worked, and Concord Farms, about a mile away.

The accused gunman, identified as Chunli Zhao, 67, was taken into custody later after he was found sitting in his vehicle, parked outside a sheriff’s station. Authorities said they believe he had come to surrender.

He killed seven and wounded one, and all of his victims were Hispanic and Asian Americans, San Mateo County Sheriff Christina Corpus told reporters on Tuesday.