FBI building evacuated for anthrax scare today. There is a 2004 Homeland Security/FBI document relating to the risk of attack on an HVAC system. Basically, it provides the advice of closing the intake if the aerosol is coming from outside and opening it if the aerosol is coming from inside. But perhaps the latter advice is misconceived — as that would risk spreading it to adjacent buildings. It seems evacuation is what you should do.
The offices were in Salt Lake.