Fire officials have not confirmed the exact cause but say the home had 40 year old cellulose insulation in the attic.  Cellulose resembles finely shredded newspaper with a fire resistant spray on it.  This article says that the spray wears off leaving an extremely flammable material.

 http://globalnews.ca/news/620333/million-dollar-fire-sparks-concern-about-old-insulation/