ORANGE, Calif.—The gunman who killed four people and critically wounded a fifth at a Southern California office building knew all the victims and apparently before opening fire chained shut the gates to two entrances, delaying police from getting inside, authorities said Thursday. Among the victims of the “horrific massacre” Wednesday afternoon was a 9-year-old boy who was found cradled in the arms of a woman believed to be his mother, Orange County District Attorney Todd Spitzer said. The woman was the only survivor among those shot. The others killed were a man and two women. “Our hearts today go out to the victims, and I’m here to tell you that we’re going to do everything in our power in the Orange County District Attorney’s office to get justice for these families,” Spitzer said. He said he will consider seeking the death penalty. The violence in the city of Orange was …