There were several moslem terrorists on the construction crew, doing menial tasks, who brought-in accelerants and remote igniters, so that when they all pulled-off the job for Easter Week, they could detonate with a cellphone.
https://www.theguardian.com/world/2016/sep/09/cell-of-french-women-radicalised-by-isis-behind-failed-notre-dame-attack
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https://www.thedailybeast.com/an-isis-plot-to-blow-up-notre-dame-cathedraland-rule-the-world
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https://www.rt.com/news/456629-french-catholic-churches-attacks/
Post #4 is my opinion.
That’s the problem with Muslims. They do not understand that Christianity does not reside in a building.....any building. Notra Dame is a testament of mans love veg for God. It is the collective outpouring of that love. A building can be destroyed but our love for God continues. Nothing, not fire, not anything can erase the millions upon millions of prayers that were offered up there. The beauty and magisty is merely the physical manifestation of our faith, nothing can touch the spiritual manifestation of that faith.
Pile of burning clothes belonged to a homeless person, my aunt fanny.
Given the scale of the fire, I think (unprofessional fire opinion) that it will be well neigh impossible to determine its cause. Which leaves intentional or unintentional.