Zhao has told investigators that he was enraged by a $100 equipment bill from his boss for damage to heavy construction equipment, which sparked the shooting spree, according to San Mateo County District Attorney Steve Wagstaffe, who confirmed an earlier report from NBC Bay Area. Shortly before the shooting, Zhao vented his frustrations at the supervisor and a co-worker whom Zhao blamed for a collision between his forklift and a bulldozer, according to the TV report later confirmed by Wagstaffe. After the confrontation, he allegedly shot the supervisor and the coworker.