That's probably DHS CBP (Customs & Border Protection) , NET 4. Also known by other channel numbers, depending on the input frequency. Inputs to this repeater could be 170.1, 166.225, 166.4375, 166.5875.
The Washington DC Secret Service field office uses this frequency also...
Were you hearing it in analog or P-25 digital?
(Ooops - I just looked at the subject line, so it was P-25)
- Chris