I would not be surprised to hear it was sparked by Islamic extremists. According to wikipedia there were two known attempts on the cathedral in the past 3 years:

> 8 September 2016: Notre Dame Cathedral bombing attempt. Arrests made after an explosives-filled car was discovered parked alongside the cathedral.

> 10 February 2017: Police arrested four people in Montpellier, France, including a 16-year-old girl and a 20-year-old man already known by authorities to have ties to extremist Islamist organizations, on charges of plotting to travel to Paris and attack the cathedral.[37]

see https://en.wikipedia.org/wiki/Notre-Dame_de_Paris#Events_in_...