Paris prosecutors are investigating if an electrical short-circuit caused the fire that ripped through Notre Dame Cathedral.
A judicial source told CNN on Thursday that "we are not excluding any hypothesis at this stage," saying that this is not their only line of inquiry.
Since Tuesday, Paris prosecutors have been investigating what could have turned the Gothic masterpiece into an inferno.
At least 40 employees at the Paris landmark have been questioned so far.
On Wednesday, the prosecutor's office said that in addition to interviews, forensics teams and the central laboratory for the police department had been able to access some areas of the site and begin inspections. Officials are continuing to pursue the theory that the cause of the fire was accidental but have not ruled out other scenarios at this stage, the prosecutor's office added.
"While the prosecutor's office does not rule out any hypothesis, we remind that at this stage, nothing in the investigations highlights a criminal origin. Accidental causes remain our privileged lead," the prosecutor's office said.
The cathedral was undergoing renovations at the time of the fire, with some scrutiny fallen on the firms undertaking work on the 150-year-old spire, which collapsed Monday as the flames raged around it.
Of the four companies contracted to carry out renovations at Notre Dame, two companies, scaffolding firm Europe Echafaudage and art conservationists Socra, had work in progress there at the time of the fire. Neither company had workers on site when the fire broke out.
The fire devastated large parts of the 850-year-old building before it was finally extinguished after a nine-hour battle.
The Paris fire service said the operation was one of the most complex it had ever undertaken, where, at one point, it was feared that the entire structure might be lost.
Scores of priceless artifacts were rescued from the flames, and were taken to the Louvre museum for safekeeping.
The cathedral was home to a selection of sacred relics including a fragment of the Wood of the Cross -- believed by many to be a part of the "true cross" on which Jesus was crucified -- and what is supposedly one of the nails that the Romans used to crucify him.
The Crown of Thorns and the Tunic of Saint Louis were among the venerated artifacts saved from the blaze on Monday.
French President Emmanuel Macron said he wants Notre Dame rebuilt in five years.
Donations toward the restoration have poured in from across the globe; the total raised so far for the reconstruction of the Gothic masterpiece has topped 800 million euros ($904 million), including gifts of tens of millions of euros from some of France's wealthiest families .
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