Modern error theory suggests that in complex systems, operator errors are the logical consequences of antecedents or precursors that had been present in the systems at the time they were committed. Whereas governments initially investigated accidents on an individual basis and assigning investigators to the investigations as they occurred, governments are establishing agencies with full-time investigative staffs exclusively to investigate accidents. Investigators should conduct investigations to learn what caused an incident or accident by establishing a link between antecedent and error, so that changes can be implemented to prevent future occurrences. Governments investigate accidents to protect the public by ensuring that the necessary steps are taken to prevent similar occurrences, mandating necessary changes to the system or, changing the nature of its oversight of the system. James Reason believes that system designers and managers place internal defenses in systems to prevent errors from leading to incidents and accidents because they recognize the potential fallibility of human performance.