The plan was foreshadowed online. Photos of the Allen mall. Pictures of guns and receipts for tactical gear. Mass shootings described as “interesting,” “sport” and “art.” It’s unclear whether the posts linked to the accused Allen mall shooter were visible to anyone but him before the attack. But even if they were, if no one reported the threats to authorities, experts say, it would have been nearly impossible for law enforcement to find them. “It’s unrealistic for the public to think that any one service, database or agency could somehow track everything in real time. That only happens in the...