It took firefighters almost half an hour before they could find the fire at the Notre Dame cathedral - even though an alarm was triggered after the roof caught fire.
Officials attending the blaze at the 850-year-old cathedral in Paris initially couldn't find any sign of fire, 23 minutes after the alarm went off.
Paris public prosecutor Remy Heitz says that firefighters were called to the blaze at 6.20pm (local time), but after an initial inspection they said they were unable to find any evidence of fire.
Firefighters remained inside Notre Dame and eventually discovered a fire in the wooden beams under the cathedral's roof, once a second alarm went off at 6.43pm.
But they couldn't put it out by then.
"The investigation is going to be long and complex," Heitz said. "We are in the process of interviewing witnesses."
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Firefighters battled for eight hours to contain the blaze, which destroyed the world-famous cathedral's roof and spire.
But despite fears the whole building could collapse, the stone structure and iconic bell towers remain standing, and the firefighters' actions meant that priceless treasures were saved from the blaze - including reputed relics of the Crucifixion.
The three irreplaceable rose windows, which date back to the 13th Century, were also intact.
Experts are warning the damage could take decades to repair.
Police have made clear that they believe the fire was an accident and renovation work may have led to the blaze.
The Paris prosecutor's office is investigating 'involuntary destruction caused by fire', treating the fire as an accident and not arson.
"Nothing suggests that it was a voluntary act," Heitz told reporters outside the Gothic cathedral.
A source close to the inquiry told The Daily Mail that many of the restorers have been interviewed, with prosecutors focusing on the equipment used high up on the medieval building.
"The fear is that a small fire began in the roof where they were working," the source said.
"The irony is that the restorers had just begun working on the spire which collapsed along with much of the roof."
More than 400 firefighters were needed to tame the inferno that consumed the roof and collapsed the spire of the eight-centuries-old cathedral.
A French cultural heritage expert claims that France no longer has trees big enough to replace the ancient wooden beams that fell during the blaze.