The National Airspace System (NAS) was restarted, albeit with new security procedures in place. Only Washington's close-in National Airport remained shuttered. The international aviation community and security services have responded to the persistent threat in a concerted effort to stay one step ahead of the terrorists. But terrorists probe for weak points in the security system, and have proved quite resourceful in adapting new methods to exploit those weaknesses. In the United States, there is no national system by which airport workers are registered and gain security access. That responsibility is conducted at the local level by the airport, with each airport implementing its own standards and methodologies for granting different levels of security access to its employees. Airlines from advanced countries, along with their respective national security services, should devote more of their attention to threats posed at airports in these regions of lax security, for those represent the weakest link in the airline's operational network.