Diplomatic Security Service
From The RadioReference Wiki
The Diplomatic Security Service (DSS) is a division of the Department of State. The DSS protects high-level diplomats when they visit the United States, except heads of state who are protected by the Secret Service.
Frequencies listed as in use nationwide except in the Washington DC area where they have different uses.
Frequency Input Type Tone Ch. Description 409.625 407.2 RM 151.4 PL 1/2 Primary - Commandpost/Motorcades 407.6 M 151.4 PL 3 Secondary - Protection Agents
Edit...The State Department now uses P25 Digital transmissions. I have verified this in both NYC and Washington D.C. Also note that most of the D.C. transmissionas are also encrypted.