It's gotta be something pretty straight-forward, like a short in some electrical receptacle, creating sparks that caused drapery, some other fabrics, carpets, or upholstery  to ignite - or a kitchen-related fire.  And then when the fire was discovered, the fire was already out-of-control and spreading - with no sprinkler system to abate its spread.  Without chemical or petroleum residue, an intense fire of this nature would destroy any evidence.  The cause cannot be rocket science - but the evidence has been incinerated.