The fact that the fire evidently began just after closing suggested arson to me, too, though the authorities haven't raised the possibility.
I hope that's not it. According to some reporters, the fire started "in the spire". If that's true, probably it was unrelated to terrorism, simply because a casual "tourist" wouldn't be able to get up there.
So far, there are no fatalities. No reported injuries, either.
They don't seem to be making much in the way of inroads on the fire in the nave. People are saying Paris has an "excellent" fire department, but I wonder how good their equipment is. Fires are less common in Europe than here.