Investigators think an electrical short circuit was the most likely cause of the Notre Dame Cathedral fire, according to a French judicial police official.
The official said investigators still do not have the green light to work in the church and search in the rubble for safety reasons.
He said the monument is still being consolidated with wooden planks to support some fragile parts of the walls.
The probe was taking place as France paid a day-long tribute to the Paris firefighters who saved the cathedral from collapse and rescued its treasures.
The tributes came as construction workers rushed to secure an area above one of the church's famed rose-shaped windows and other vulnerable sections of the fire-damaged landmark.Courage
French President Emmanuel Macron held a ceremony at the Elysee Palace to thank the hundreds of firefighters who battled the fast-moving fire for nine hours on Monday.
"We've seen before our eyes the right things perfectly organised in a few moments, with responsibility, courage, solidarity and a meticulous organisation", he said, adding: "The worst has been avoided."
Mr Macron said the firefighters would receive a medal for their courage and devotion.
The huge cathedral was in the initial stages of a lengthy restoration. The roof and spire were destroyed, but Notre Dame's famous bell towers, rose windows, organ and precious artworks were saved.
Fire officials warned that the building remains very fragile and extremely dangerous for construction workers, restoration experts and neighbours.
Police, citing "important risks" of collapse and falling objects, officially closed a large swathe of the island in the Seine on which Notre Dame sits. The area had been unofficially blocked off since the fire.
Workers using a crane removed some statues to lessen the weight on the cathedral's fragile gables, or support walls, and to keep them from falling.
Remarkably, no one was killed in the fire, which began during a mass, after firefighters and church officials speedily evacuated those inside.
Mr Macron wants to rebuild the cathedral within five years - in time for the 2024 summer Olympics that Paris is hosting.