Based on the ultimate force program...

Counter terrorists: The terrorists have got the anthrax inside the building, and the scientists working on it are held hostage. If the anthrax container is broken, it will contamonate the air and kill hundreds of thousands of people. Just rescue the hostages or wipe out the targets.
Terrorists: Hold the hostages in the building, and kill any one in your way. If that fails, shoot the anthrax...