Probably natural gas, at least initially. There may have been secondary dust explosions. To have that big of a dust explosion is unlikely because a very large amount of the building would have to be filled with a lot suspended dust. I know flammable dust explosions can be huge, but there were workers in those spaces so there would have to be significant control methods even if they weren't following safety rules. They probably wouldn't be able to work if there was that flour and or sugar dust in the air.