Assuming they had a security camera system that probably should have had remote monitoring capability, they should have been able to locate the general location where the fire started if it was in a "public" area.
If it started in a construction area, they need to figure out if it was an inside job. Hopefully the church wasn't dumb enough to hire jihadis to renovate the place.
A good fire investigator shouldn't have much trouble determining the place where the fire started with all of the video footage and other evidence.
Never assume.
It is reported several times, it started in a work area in the steeple, where the ancient roof timbers fueled the fire like heart pine lider, and ran the whole roof timber area, but not into the lower areas.
A few sources have reported Macron is catching flack because there were no sprinklers in the roof area, and that lack of preventive equipment is missing in other historic sites as well, and has been reported as problems in previous historical site's fires.
These reports have been squelched. It was "initially believed" to be a construction accident in the steeple.
All that has been drowned in the conjecture and following news where more questions are asked than facts presented and the conjecture comes faster than investigations can produce results.
Fwiw, I did fire investigation for 8 years, and have seen really hot fires destroy evidence, and it come down to a single witness "finally" telling, I screwed up, sometimes years later.
We are going to have to wait and see, then judge the evidence, not conjecture or opinion.
Best to you all.