It only takes a small opening for soot to travel into areas of an attic or interior walls during a fire.  This photo is an example of how much soot got into the attic above the office area of this shop fire.  The top side of the batt insulation is completely black. 

This is the aftermath of a fire in a condo living room.  The home suffered from signifiant smoke and soot damage.  SERVPRO was able to respond and clean the damages from the home.