Authorities suspect that faulty wiring or a burning cigarette may have caused the devastating blaze, but they cannot say with certainty. Still, they are confident that the fire was not intentionally lit.
Prosecutors in Paris said on Wednesday that a preliminary investigation into the origins of the April 15 blaze that destroyed the roof and spire of the city's landmark medieval Notre Dame Cathedral did not suggest criminal activity.
"For now there are no indications of a criminal origin," Paris prosecutor Remy Heitz said in a statement. Heitz added, "The investigations carried out to date have not yet been able to determine the causes of the fire."
The preliminary investigation, based on 100 witness interviews, did not draw specific conclusions as to the cause of the fire but suggested faulty electrical wiring or a smoldering cigarette may have sparked the blaze.
Heitz's office said that "further investigations, using significant expertise, will now be undertaken."
The ongoing investigations will be led by three magistrates, all of whom will have the power to press charges against individuals or companies determined to be negligent.
Although he admitted that employees occasionally smoked on site, he said, "no way could a cigarette butt be the cause of the fire at Notre Dame."
The devastating fire began on the evening of April 15, destroying the roof and spire of the building before firefighters could put it out hours later.
Though the building's historical bell towers and facade were saved, the structure is considered fragile. That fact, along with contamination from large amounts of lead from the melted roof, has slowed clean-up work.