For many years, until December 1977, the Agency's first line of defence, beyond the triple fence, in an overall program known as "security in depth" was the Marines. Activated on October 15, 1954, as the first (and only) Marine Guard unit ever assigned to a national intelligence organization, they manned the gatehouses surrounding SIGINT City, checking badges and sorting through briefcases, purses, and lunch bags. For five hours before they took up their guard positions each day, they would form what was known as an Alert Force and remain quartered in a Marine barracks three hundred yards behind the Operations Building. In the event of a threat to security, they would grab their helmets and weapons and rush to the trouble spot.
Beyond the gatehouses, a dizzing system of magnetic, color-coded, key-punched badges determines where in the buildings a person is allowed to go. Those fully cleared and with the requisite "need to know" wear the green badges granting them access to all but specially compartmented spaces. Yellow is reserved for non-SIGINT organizations with offices in the Agency. For those holding anything less than a full clearance, red badges are issued.
And much more in the actual book...............