The scenario was this: a terrorist with a backpack full of sodium cyanide--a chemical used in gold mining operations that quickly attacks and shuts down the human respiratory system--unleashed it inside 555 California.
The attack instantly kills a number of people and injures others.
Because of the victims, the fire department is the first to arrive on the scene, and when the firefighters discover what's happened, they isolate the building's lobby and deny entry into the building by anyone else, and then quickly set up a mass decontamination system nearby. And then they call in the specialists.
This is, of course, the whole point: to bring in the many different types of specialists to deal with the variety of complexities a situation like this creates.
But to make it even more complicated, the scenario had more to it: emergency personnel also discover an improvised explosive device inside 555 California, and just when they're dealing with everything going on there, there's also a shooting in another office building nearby.