“A short-circuit might have caused the fire. Some suggested causes of the fire include a short-circuit from the spire’s electric bells, or an elevator, or cigarette butts left by workers.”
https://en.wikipedia.org/wiki/Notre-Dame_de_Paris_fire#Investigation
My money is on a “Center Fuel Tank Explosion”
Here's my issue.
The fire department was alerted by an alarm. When they arrived they searched the building and apparently found neither a fire or smoke. They left only to return later to a raging fire.
WHAT DID THEY MISS? How complete was the search that they didn't find smoldering, a fire, or smoke?
Either they were sloppy in their search or someone was in the building when they left and started the fire.