Webcam footage has surfaced of a person moving around on the roof before the fire started, with a flash next to them. Time-lapse footage from the same webcam shows that around where the flash appeared, is where the fire started.
Theories of arson/terrorism have been widespread, the government of France said it was a likely accident before the fire was even put out. We know the French government covered up some of the horrors of the Bataclan theatre attack to prevent a backlash against the French Muslim community already.
Here is the video, from a webcam watching the roof. You can see a person walking around, seemingly without a high visibility vest(very against codes and could get the whole site shut down if it were a worker), then a flash where they are standing:
The webcam seems to have been taking short clips 5 minutes after every hour, and in this image you can see that the fire started where the flash was:
The French government lied about the torture by Muslim attackers inside the Bataclan to families of victims to prevent intolerance towards multiculturalism. Of course, they would lie about an 800-year-old cathedral being torched. Working on an old building has tons of regulations to keep an accidental fire from starting. And why did government agencies start saying it was an accident before the fire was even out? They know that if the French people start to think this was arson started by foreigners, the status quo of letting them come in by the thousands will not be acceptable. The yellow vests have shown that the French are willing to act against corrupt leaders. This fire may become one that lights a very large powder keg.