I think it was a natural disaster.
Here you've got one of America's biggest cities, and then all of a sudden, a lot of monsters make one massive, unified effort to destroy everything, and in six days the city is destroyed? What, is this some kind of monster war? Unless we're dealing with extremely intelligent creatures that are in a legion against Chicago, I'm pretty sure it's going to be just random, separate, uncoordinated attacks against a city. Which means that it would take a lot more than six days to do that kind of damage. You don't just destroy Chicago in six measly days unless it's a unified effort, not random.
But on the other hand, nature can easily do that kind of work in that amount of time. Easily. Viruses, diseases, all that stuff spreads like wildfire.
My ideas: if this game has some sort of monster race in it, they have be intelligent, they must be coordinated, which means we're tied up in some sort of Halo/Gears of War scenerio.
If this game is based off of purely natural disasters, then we're tied up in a personal struggle to survive.