paragraph: On 9/11, the defense of U.S. airspace depended on close interaction between two federal agencies: the FAA and the North American Aerospace Defense Command (NORAD). The most recent hijacking that involved U.S. air traffic controllers, FAA management, and military coordination had occurred in 1993.90 In order to understand how the two agencies interacted eight years later, we will review their missions, command and control structures, and working relationship on the morning of 9/11. FAA Mission and Structure. As of September 11, 2001, the FAA was mandated by law to regulate the safety and security of civil aviation. From an air traffic controller's perspective, that meant maintaining a safe distance between airborne aircraft. Many controllers work at the FAA's 22 Air Route Traffic Control Centers. They are grouped under regional offices and coordinate closely with the national Air Traffic Control System Command Center, located in Herndon, Virginia, which oversees daily traffic flow within the entire airspace system. FAA headquarters is ultimately responsible for the management of the National Airspace System. The Operations Center located at FAA headquarters receives notifications of incidents, including accidents and hijackings. FAA Control Centers often receive information and make operational decisions independently of one another. On 9/11, the four hijacked aircraft were monitored mainly by the centers in Boston, New York, Cleveland, and Indianapolis. Each center thus had part of the knowledge of what was going on across the system. What Boston knew was not necessarily known by centers in New York, Cleveland, or Indianapolis, or for that matter by the Command Center in Herndon or by FAA headquarters in Washington. Controllers track airliners such as the four aircraft hijacked on 9/11 primarily by watching the data from a signal emitted by each aircraft's transponder equipment. Those four planes, like all aircraft traveling above 10,000 feet, were required to emit a unique transponder signal while in flight. On 9/11, the terrorists turned off the transponders on three of the four hijacked aircraft. With its transponder off, it is possible, though more difficult, to track an aircraft by its primary radar returns. But unlike transponder data, primary radar returns do not show the aircraft's identity and altitude. 
question: On 9/11/01 what was the FAA mandated to do by law and what did that mean for air traffic controllers?
answer: Regulate safety and security of civil aviation and air traffic controllers do that by maintaining a safe distance from airborne aircraft