From Wikipedia:
FPCON NORMAL describes a situation of no current terrorist activity. The only security forces needed are enough to stop the everyday criminal, similar to civilian police forces.
FPCON ALPHA describes a situation where there is a small and general terrorist activity that is not predictable. However, agencies will inform personnel that there is a possible threat and standard security procedure review is conducted.
FPCON BRAVO describes a situation with somewhat predictable terrorist threat. Security measures taken by agency personnel may affect the activities of local law enforcement and the general public.
FPCON CHARLIE describes a situation when an instance occurs or when intelligence reports that there is terrorist activity imminent.
FPCON DELTA describes a situation when a terrorist attack is taking place or has just occurred. FPCON DELTA usually occurs only in the areas that are most vulnerable to or have been attacked. One example is the September 11, 2001 Attacks.
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