Condo Unit on Fire

In December 2017, a condo unit in BGC caught fire.

By mid-day, photos of a burning unit surfaced on Facebook. The whole unit caught fire. The firemen broke down the door and put out the fire before it could damage adjacent units. By then, almost everything inside the unit was reduced to ashes. According to the firemen, the culprit was the AC which was left on while the tenant was away. It also looked old–older than the unit. They said the cause for the fire was the AC, which was left on (which the tenant denies).

So the questions are: was it the tenant's fault for leaving the AC on while he/she was away?

Or was it the owner's since they used an old (and it seemed faulty) AC?

Was the unit insured? Note that the building's fire insurance doesn't cover items found in the units.

Who will pay for what?

What's the lesson here?