The area had interoperability after Air Florida flight 90 crash. Several years later most public safety agencies in the DC area had full radio interoperability with each other. I said most because when you get involved with the federal government they usually don't want to cooperate with non federal agencies unless they thought up the idea first and can control everything. (soapbox off).
When some DC area agencies started talking ABOUT ENCRYPTION and implementing same it totally destroyed the interoperability that had been built in. You saw the effects of this when DC fire department went encrypted after the PD and then during mutual aid for a metro fire/smoke incident couldn't communicate with neighboring FD's on scene. Mayor Bowser ordered the encryption dropped the next day which it was (good for her). When they had the Navy Yard shooting, DC requested Fairfax County PD helicopter for operational support, but because DC is encrypted the Fairfax helicopter couldn't t communicate with the DC officers on scene. Encryption blows up everything!
So now Fairfax County PD radio's have the encryption keys for the agencies that are encrypted with a few exceptions. US Cap PD is one of the exceptions. There are a few others out there but encryption has hamstrung what once was the envy of public safety agencies around the country, when during 9/11 and the attack on the Pentagon all public safety agencies that responded had full radio interoperability because none of them were using encryption at that time!
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