What is the controlling agency in unactivated Special Use Airspace (SUA)?

In the context of unactivated Special Use Airspace (SUA), the controlling agency is indeed the ATC facility managing the airspace. When SUA is not active, it becomes available for general aviation use, and the air traffic control facility responsible for that airspace retains management authority. This means that they will provide services like air traffic separation and advisories in that area.

While the military unit may hold responsibility for the SUA when it is activated, during times when it is not in use, they do not have control over the airspace. The FAA provides the regulations and framework within which the airspace operates, but it is the ATC facility that actively manages the airspace and coordinates air traffic as needed. Similarly, local air traffic control towers manage traffic in their vicinity but do not have control over SUA when it is unactivated, as their jurisdiction is typically limited to the airspace immediately around the airport.