French investigators were examining many hypotheses "including a malfunctioning of the electrical system or a fire which started with a badly stubbed-out cigarette", said a statement, indicating there was no evidence to back up any theory of "a criminal origin" to the fire.
The statemen by chief Paris prosector Remy Heitz, said the preliminary conclusions had been based on interviews with some 100 witnesses.
He emphasised that the investigation had still not clarified the actual cause of the fire. It was not yet possible to conclude if either an electrical fault or stubbed-out cigarette was the most likely theory, the report says added.
"Deeper investigations, using significant expertise, will now be undertaken," it said.
The statement said a preliminary investigation had now been opened, without targeting any single individual, over involuntary damage caused by negligence.
In April, a spokesman for the scaffolding company which had been involved in restoration work in the cathedral admitted that workers had smoked on the site from time to time.
Notre-Dame was gutted by a fire on 15 April that felled its steeple and consumed the structure of beams supporting the roof.
The damage to the world heritage landmark shocked the world, with French President Emmanuel Macron setting an ambitious target of five years to restore the edifice.
Pledges of some 850 million euros to finance the reconstruction were made by prominent French businessmen and ordinary citizens, but only 10 percent of that money has been donated so far.