Investigators believe an electrical short circuit was the most likely cause of the Notre-Dame fire, it has been revealed.
The fire on Monday tore through the Parisian landmark, destroying much of the cathedral's interior and causing the spire to come crashing down.
A police official has said investigators still do not have the go-ahead to work in the cathedral for safety reasons, adding that the monument is currently being reinforced with wooden planks to support some parts of the structure.
The fire service have warned that the building remains fragile and extremely dangerous for people to work in.
Police have also closed off a large part of the island which the cathedral sits on, citing "important risks" of collapse and falling objects.
President Emmanuel Macron held a ceremony at the Elysee Palace to thank the firefighters who saved the building from total collapse.
Revealing that the firefighters will receive a medal for their "courage and devotion", Mr Macron said: "We've seen before our eyes the right things perfectly organised in a few moments, with responsibility, courage, solidarity and a meticulous organisation.
"The worst has been avoided."
It was previously thought that the fire was a result of the renovation work going on, though investigators confirmed that no one was on site as the fire broke out.
It is believed that the fire was started accidentally and investigators are questioning cathedral staff and construction workers. Around 40 people have been spoken to so far.
Firefighters say Notre-Dame came within 15 to 30 minutes of total destruction.
Hundreds of millions of euros has been pledged by businessmen and philanthropists to rebuild Notre Dame.
President Macron vowed to restore the cathedral within five years, however experts say it could take up to 15 years.