Most major agencies have their own radio operators. Some smaller agencies will use a larger agency's system. For example, this is common with the CBP vhf system where smaller agencies like National Marine Fisheries, Social Security OIG, etc. will subscribe to that system. Howerver, now with the use of cell phones and wireless laptops by law enforcement, a lot of traffic is not being heard on the air these days.
I agree with dave3825. A lot of federal agencies have the local pd's frequencies programmed in their radios. That used to be the case in south Florida before most of the locals went to 800 MHz.