Investigators suspect an electrical short-circuit was the most likely cause of the Notre Dame cathedral fire, according to a French judicial police official.
The official, who did not wish to be named, says investigators still do not have the green light to work in the cathedral or 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.
It took firefighters over 12 hours to extinguish an inferno engulfing Paris' iconic Notre Dame. Source: AAP
The probe was taking place as France paid a day-long tribute to the Paris firefighters who saved the famous 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.
President Emmanuel Macron held a ceremony at the Elysee Palace on Thursday to thank the hundreds of firefighters who battled the fast-moving blaze at Notre Dame for nine hours, preventing its destruction and rescuing many of the important relics inside.
"We've seen before our eyes the right things perfectly organised in a few moments, with responsibility, courage, solidarity and a meticulous organisation", Macron said.
"The worst has been avoided."
Firefighters will receive a medal for their courage and devotion, the president said.
The huge cathedral, including the spire that was consumed by flames and collapsed, had been in the initial stages of a lengthy restoration.
The roof was 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.
Remarkably no one was killed in the blaze - which began during a Mass - after firefighters and church officials speedily evacuated those inside.
Among the firefighters honoured was Paris fire brigade chaplain Jean-Marc Fournier, who said he was falsely credited with helping salvage the crown of thorns believed to have been worn by Jesus at his crucifixion.
The chaplain said a team of rescuers broke the relic's protective covering and an official who had the secret code to unlock the protection finished the job.
Investigators so far believe the fire was accidental, and are questioning both cathedral staff and workers who were carrying out renovations.
Some 40 people had been questioned by Thursday, according to the Paris prosecutor's office.
Macron wants to rebuild the cathedral within five years - in time for the 2024 Olympics that Paris is hosting.
However experts say the vast scale of the work to be done could easily take 15 years, since it will take months, even years, just to figure out what should be done.
