Insignia Description
A gold color metal and enamel device 1 5/32 inches (2.94 cm) in height overall consisting of a shield blazoned as follows: Argent, on a pale emitting in saltire four lightning flashes Azure (Teal Blue) a key bit to dexter in base, the bow a bear’s head, Or. Attached above the shield is a Gold triparted scroll inscribed “SENTINELS OF SECURITY” in Black letters.

Symbolism
Teal blue and silver refer to the colors formerly used for the U.S. Army Security Agency. The key symbolizes the unit’s mission which is providing security. The golden bear’s head on the key represents California where the unit activated. The lightning flashes, symbolizing electricity, relate to the importance of electronic communications as part of the unit’s functions.

History
The distinctive unit insignia was originally approved for the 309th Army Security Agency Battalion, Army Reserve on 12 February 1959. It was assigned for use by the 325th U.S. Army Security Agency Battalion on 5 August 1959. It was reassigned for the 309th U.S. Army Security Agency Battalion on 2 August 1965. The insignia was redesignated for the 309th Military Intelligence Battalion on 2 May 1990.

Source: The US Army Institute of Heraldry