Riddle me this, riddle me that...

Are zombies something of a common occurrence in this particular 'reality'?

I ask because of the mall's immediate lockdown protocol, or are they going off of an 'active shooter' scenario? Even then, it's doubtful that Mall Security would want to lock a bunch of innocent people in with an armed gunman or gunmen. It would be an absolute slaughter. 

How does the outbreak occur? 
What kind of model are you focusing on for incubation times?
Are you going with fast zombies, regular zombies, both.. or an even wider array of zombies?

How big is the mall? 
How much freedom and control do you plan for the players to have?
How 'crunchy' do you plan on the zombies being? (ie how tough)

Do humans go good with ketchup?