Well, an ifo which has not been released in the media concerning ND :
The 15th of april, both towers of ND were exceptionnaly closed at 5:30 pm instead of usually 6:30 pm.
First fire alarm started at 6:20 pm and the second at 6:43 pm. Fire has been constated at around 6:50 pm.
What a coincidence that the fire started precisely between 5:30 pm and 6:20 pm, when both towers were exceptionnaly closed to public.
http://www.panamza.com/190419-notre-dame-exceptionnellement/
http://www.wikistrike.com/2019/04/notre-dame-les-tours-etaient-exceptionnellement-fermees.html