Police investigating the Notre Dame fire are looking into why smoke detectors failed to go off, reports say.
The cathedral, which is currently undergoing construction work, went up in flames on Monday April 15.
French company Le Bras Frères, who are one of five firms working on Notre Dame, said alarms on the scaffolding surrounding the buildings spire did not sound.
Marc Eskenazi, head of communications at the company, told French newspaper The Connexion that detectors only went off inside the cathedral itself.
He added that the company were working alongside police with their inquiry.
Investigators have now been able to establish that the fire likely started at the centre of the cathedrals roof, towards the base of the iconic spiral, CNN reported.
Officers are said to have reviewed numerous amateur videos and photos captured by those who first saw the smoke rising from the Paris landmark.
The network also reported that security guards told police they only noticed the flames for the firs time when they were already three metres high at 6.43pm.
This could have been due to a smoke alarm failure, they said.
Firefighters were able to leave Notre Dame last night after architects and construction workers finished stabilising its damaged structure.