Speculation about an arson attack has been investigated during five years of forensic analysis, but no evidence was found.
The current chief Paris prosecutor Laure Beccuau said in April that "the closer we have got to the spot the fire started, and the more results of analyses come back, the more weight is lent to the theory of an accident."
Fee row
Culture Minister Rachida Dati has proposed that visitors to the restored cathedral pay a five-euro ($5.25) entry ticket, with the funds set to be routed to some 4,000 churches in need of repairs around France.
Charging for entry -- entering Notre Dame was previously free -- would bring the tourist attraction into line with St Paul's cathedral in London or Milan's Duomo.