Columbia subject headings:
Airports--Security measures--United States.
Intelligence service--United States.
Public buildings--Security measures--United States.
Terrorism--United States--Prevention.
Undercover operations--United States.
Harvard subject headings:
Administrative agencies--Security measures--United States.
Airports--Security measures--United States
Executive departments--Security measures--United States.
Intelligence service--United States.
Public buildings--Security measures--United States.
Terrorism--United States--Prevention
Undercover operations--United States
Princeton subject headings:
Airports--Security measures--United States.
Intelligence service--United States.
Public buildings--Security measures--United States.
Terrorism--United States--Prevention.
Undercover operations--United States.