Officials attending the Notre Dame blaze initially couldn't find any sign of fire despite the alarm being raised 23 minutes before the structure went up in flames.
Paris public prosecutor Rémy Heitz announced yesterday that firefighters were called to the world-famous cathedral at 6.20pm CET, almost half an hour before the roof caught fire.
But after an initial inspection they said they were unable to find any evidence of fire.
After a second alarm was raised a shortly while later, firefighters attending the scene said they were unable to stop the blaze from ripping through Notre Dame's wooden roof structure.
Heitz said this evening: 'What we know at this stage is that there was an initial alarm at 6.20pm, followed by a procedure to verify this but no fire as found.
"Then, there was a second alarm at 6.43pm and at that point a fire was detected in the structure.
"The investigation is going to be long and complex," he added. "We are in the process of interviewing witnesses."
After eventually responding to the blaze, French firefighters were filmed racing into the burning Notre Dame to save the 850-year-old cathedral from destruction.
The fearless rescue team charged into the inferno with huge flames leaping across the church and sparks tumbling from the ceiling after the roof of the Paris landmark caught fire.
Firefighters battled for eight hours to contain the blaze which destroyed the cathedral's roof and spire, but despite fears that the whole building could collapse the stone structure and iconic bell towers remain standing today.
The firefighters' actions - and their quick deployment of a robot to hose down the cathedral - meant that priceless treasures including reputed relics of the Crucifixion and 13th-century stained-glass windows were saved from the blaze.
Today haunting new photos of Notre Dame's interior showed debris strewn across the floor - as experts warned the damage could take decades to repair - while detectives probe the renovation work which may have led to the blaze.
The pictures revealed that a statue of Jesus descending from the cross, positioned on the altar, was preserved among the smouldering rubble after the roof was destroyed by Monday night's fire.
The three "irreplaceable" Rose Windows, which date to the 13th century and were feared to have melted or exploded, were also intact.
Attention has now turned to what may have caused Notre Dame, part of which was being restored in a €150million refurbishment, to fall victim to such a disaster.
The focus of prosecutors is currently on contractors Le Bras Freres, whose owner Julien Le Bras last year boasted his firm's goal was "to keep as many old items as possible and not to put the building at risk" after they were awarded a €5.7m contract as part of the cathedral's restoration project.
Detectives investigating the catastrophic blaze are interviewing specialist restorers who were carrying out works on the cathedral spire when the inferno broke out.
French police are understood to have launched a criminal inquiry after a "stray flame" caused fire to engulf the landmark last night, with heroic firefighters battling for eight hours to bring the blaze under control.
The Paris prosecutor's office is probing "involuntary destruction caused by fire", indicating authorities are treating the blaze as a tragic accident and not arson or terrorism.
"Nothing suggests that it was a voluntary act," Remy Heitz told reporters outside the Gothic cathedral, adding that the workers employed at the site were being questioned over the blaze.