Sorry for the misfortune! If everything was pretty well burned up, sometimes that makes it more difficult to determine an actual cause. Obviously it's usually in the engine compartment, but doesn't have to be.

I went to a car fire one day and it was only slight damage to the inside of the car, right behind the driver's seat. At first we couldn't figure out what could have started a fire right behind the drivers seat. Once we started questioning the driver some more, she mentioned throwing a cigarette butt out of her window as she was driving to class that morning. What she didn't realize was that the butt came right back in the window behind her and landed on the floor board. She goes off to class only to find us there squirting water in her back seat to put the fire out. Found the cigarette butt during our overhaul. Crazy.